A U.N. team leaves its Damascus, Syria, hotel in a convoy on Monday, August 26. The team was to investigate an alleged chemical attack that killed hundreds last week in a suburb of the Syrian capital. Sniper fire hit a vehicle used by the U.N. chemical weapons investigation team multiple times Monday, according to the United Nations. Tensions in Syria began to flare in March 2011 and have escalated into an ongoing civil war. Click through to view the most compelling images taken since the start of the conflict. 
A Syrian soldier walks down a street in Damascus on Saturday, August 24. 
Pigeons lie dead on the ground on August 24 from after what activists say is the use of chemical weapons by government forces in the Damascus suburb of Arbeen. 
Columns of smoke rise in Barzeh after heavy shelling on Friday, August 23. 
A young Free Syrian Army fighter is reflected in a mirror as he takes position in a house in Aleppo on Thursday, August 22. 
Rebels move around a building in Aleppo on August 22. 
Syrian rebels claim pro-government forces used chemical weapons to kill citizens outside Damascus on Wednesday, August 21. People inspect bodies in this photo released by the Syrian opposition Shaam News Network. 
People search the rubble of a bombed building in Aleppo, Syria, on Friday, August 16. 
Men bury the bodies of six members of the same family killed in a bombing in Raqqa on Saturday, August 10. 
Syrian Army soldiers patrol a devastated street in Homs on Wednesday, July 31. 
Free Syrian Army fighters move through a hole in a wall in Khan al-Assal on Monday, July 22, after seizing the town. 
A rebel fighter walks past swings in a deserted playground in Deir al-Zor, Syria, on Sunday, July 21. 
A rebel fighter speaks with a fellow fighter through a hole in a wall in Deir al-Zor on July 21. 
A Free Syrian Army fighter casts a shadow on a wall as he carries his weapon in a shelter in Deir al-Zor on Thursday, July 18. 
Yahya Sweed, 13, is comforted by his father as he lies on a bed in Kfar Nubul on Tuesday, July 16. The boy was injured by shrapnel, resulting in the amputation of his right leg. 
A rebel fighter naps in a trench about 300 feet from the Syrian government forces' positions along the highway connecting Idlib with Latakia on Monday, July 15. 
A rebel fighter uses a hole in the wall of a destroyed school to aim at Syrian government forces in the Izaa district of Aleppo on Sunday, July 14. 
A Free Syrian Army fighter uses a mirror to scope out snipers loyal to Syrian President Bashar al-Assad in Aleppo on Friday, July 12. 
A Free Syrian Army fighter stands over a boy who was injured during shelling in Al-Bara on Monday, July 8. 
Members of the Free Syrian Army fire a homemade rocket toward regime forces in Deir al-Zor on Sunday, June 16. 
Syrian rebels leave their position in the northwestern town of Maaret al-Numan on Thursday, June 13. The White House said that the Syrian government has crossed a "red line" with its use of chemical weapons and announced it would start arming the rebels. 
Forces loyal to Syrian President Bashar al-Assad are seen near Qusayr on Thursday, May 30. 
Syrian rebels take position in a house during clashes with regime forces in the old city of Aleppo on May 22. 
Syrian army soldiers take control of the village of Western Dumayna north of the rebel-held city of Qusayr on Monday, May 13. Syrian troops captured three villages in Homs province, allowing them to cut supply lines to rebels inside Qusayr town, a military officer told AFP. 
Rebel fighters fire at government forces in the northern Syrian city of Aleppo on Sunday, May 12. 
Searchers use a flashlight as they look for survivors among the rubble created by what activists say was a missile attack from the Syrian regime, in Raqqa province, Syria, on April 25. 
A Kurdish fighter from the "Popular Protection Units" (YPG) takes position inside a building in the majority-Kurdish Sheikh Maqsood area of Aleppo, on Apri. 21. 
People walk past destroyed houses in the northern Syrian town of Azaz on Sunday, April 21. 
Free Syrian Army fighters take positions prior to an offensive against government forces in the Khan al-Assal area, near Aleppo on Saturday, April 20. 
Men inspect damage at a house destroyed in an airstrike in Aleppo on April 15. 
Syrian and Kurdish rebel fighters walk in the Sheikh Maqsud district of Aleppo on April 14. 
A female rebel monitors the movement of Syrian government forces in Aleppo's Sheikh Maqsud neighborhood on Thursday, April 11. 
A rebel runs to avoid sniper fire from Syrian government forces in Aleppo on April 11. 
Syrian rebels observe the movement of Syrian government forces around Al-Kendi hospital in Aleppo on Wednesday, April 10. 
Rescue teams and security forces check out the scene of a deadly car bomb explosion in Damascus on April 8. 
The fighting has taken a toll on buildings in Aleppo's Saladin district, seen here on April 8. 
A Syrian rebel runs for cover in Deir ez-Zor on April 2. 
A rebel checks for snipers across the street toward the Citadel in Aleppo, Syria, on Saturday, March 30, in this photo taken by iReporter Lee Harper. 
A Free Syrian fighter mourns the death of a friend in Aleppo on March 30, in this photo taken by iReporter Lee Harper. 
A Syrian opposition fighter runs for cover from Syrian army snipers in Aleppo on Wednesday, March 27. 
A Syrian girl covers her face to protect herself from fumes as a street covered with uncollected garbage is fumigated in Aleppo on Sunday, March 24. 
A Syrian man and his family drive past damaged buildings in Maarat al-Numan, on Wednesday, March 20. 
Syrians carry the body of a Syrian army soldier during a funeral ceremony in Idlib province on Tuesday, March 19. 
Syrian rebels take position in Aleppo, the largest city in the country, on March 11. 
Syrian men search for their relatives amongst the bodies of civilians executed and dumped in the Quweiq River on March 11. 
A Free Syrian Army fighter looks back as smoke rises during fighting between rebel fighters and forces loyal to Syria's President Bashar al-Assad on the outskirts of Aleppo on Saturday, March 2. 
Residents read Shaam News newspapers published by the Free Syrian Army in Aleppo on March 2. 
A member of the Free Syrian Army reacts to the death of a comrade who was killed in fighting, at Bustan al Qasr cemetery in Aleppo on Friday, March 1. 
A rebel fighter throws a home-made grenade at Syrian government forces in Aleppo on February 16. 
A member of the Free Syrian Army stands with his weapon as he looks at a rainbow in Aleppo on February 16. 
A Syrian woman looks through a bus window in Aleppo on February 14. 
Free Syrian Army fighters walk through a dust-filled stairwell in Damascus on February 7. 
A Syrian rebel gestures at comrades from inside a broken armored personnel carrier in Al-Yaqubia on February 6. 
A rebel fighter throws a hand grenade inside a Syrian Army base in Damascus on February 3. 
People stand in the dust of a building destroyed in an airstrike in Aleppo, Syria on February 3. 
Free Syrian Army fighters run as they enter a Syrian Army base during heavy fighting in the Arabeen neighborhood of Damascus on February 3. 
An unexploded mortar shell fired by the Syrian Army sits lodged in the ground in Damascus on January 25. 
Fighters from Fateh al Sham unit of the Free Syrian Army fire on Syrian Army soldiers at a check point in Damascus on January 20. 
A Free Syrian Army fighter walks between buildings damaged during Syrian Air Force strikes in Damascus on January 19. 
A Syrian rebel fighter tries to locate a government jet fighter in Aleppo on January 18. 
Syrian rebels launch a missile near the Abu Baker brigade in Albab on January 16. 
A Syrian boy walks near rubbish next to tents at a refugee camp near the northern city of Azaz on the Syria-Turkey border, on January 8. 
Syrians look for survivors amid the rubble of a building targeted by a missile in Aleppo on January 7. 
A father reacts after hearing of a shelling by forces loyal to Syria's President Bashar al-Assad in Aleppo on January 3. 
A patient smokes a cigarette at Dar Al-Ajaza psychiatric hospital in Aleppo on December 18, 2012. The psychiatric ward, housing around 60 patients, has lacked the means to function properly since fighting broke out there in July. 
Syrians mourn a fallen rebel fighter at a rebel base in the al-Fardos area of Aleppo on December 8. 
Members of Liwa (Brigade) Salahadin, a Kurdish military unit fighting alongside rebel fighters, monitor the area in the besieged district of Karmel al-Jabl in Aleppo on December 6. 
A member of Liwa Salahadin aims at a regime fighter in the besieged district of Karmel al-Jabl in Aleppo on December 6. 
Two young boys sit underneath a washline in a refugee camp on the border between Syria and Turkey near Azaz on December 5. 
The bodies of three children, who were allegedly killed in a mortar shell attack that landed close to a bakery in Aleppo, on December 2, are laid out for identification by family members at a makeshift hospital at an undisclosed location of the city. 
Smoke rises in the Hanano and Bustan al-Basha districts in Aleppo on December 1 as fighting continues through the night. 
Damaged houses in Aleppo are seen after an airstrike on November 29. 
A Syrian rebel mourns the death of a comrade in Maraat al-Numan on November 20. 
Syrians protesters stand on Assad's portrait during an anti-regime demonstration in Aleppo on November 16. 
A Syrian rebel takes cover during fighting against Syrian government forces in Aleppo on November 15. 
Syrian opposition fighter Bazel Araj, 19, sleeps next to his pistol in Aleppo on November 11. 
A rebel fighter fires at a Syrian government position in Aleppo on November 6. 
A Syrian rebel leaps over debris left in the street while running across a "sniper alley" near the Salahudeen district in Aleppo on November 4. 
Rebels hold their position in the midst of a battle on November 3 in Aleppo. 
A man cries while being treated in a local hospital in a rebel-controlled area of Aleppo on October 31. 
A man is treated for wounds after a government jet attacked the Karm al-Aser neighborhood in eastern Aleppo on October 31. 
A Syrian rebel interrogates a handcuffed and blindfolded man suspected of being a pro-regime militiaman in Aleppo on October 26. 
Smoke rises from a fuel station following a mortar attack as Syrian women walk on a rainy day in the Arqub neighborhood of Aleppo on October 25. 
A Syrian rebel fires at an army position in the Karm al-Jabal district of Aleppo on October 22. 
A wounded Syrian boy sits on the back of a truck carrying victims and wounded people to a hospital following an attack by regime forces in Aleppo on October 21. 
A man lies on the ground after being shot by a sniper for a second time as he waits to be rescued by members of the Al-Baraa Bin Malek Battalion, part of the Free Syria Army's Al-Fatah brigade, in Aleppo on October 20. 
Syrian army soldiers run for cover during clashes with rebel fighters at Karam al-Jabal neighborhood of Aleppo on October 20. 
Smoke rises after a Syrian Air Force fighter jet fired missiles at the suburbs of the northern province of Idlib on October 16. 
A Syrian opposition fighter stands near a post in Aleppo on October 11. 
A Syrian man mourns the death of his father, who was killed during a government attack in Aleppo on October 10. 
A rebel fighter is carried by his friends and laid on a gurney to be treated for gunshot wounds sustained during heavy battles with government forces in Aleppo on October 1. 
Syrian rebels help a wounded comrade to an Aleppo hospital after he was injured in a Syrian army strike on September 18. 
Free Syria Army fighters are reflected in a mirror they use to see a Syrian Army post only 50 meters away in Aleppo on September 16. 
A Syrian man carrying grocery bags tries to dodge sniper fire as he runs through an alley near a checkpoint manned by the Free Syria Army in Aleppo on September 14. 
A woman walks past a destroyed building in Aleppo on September 13. 
Free Syrian Army fighters battle during street fighting against Syrian army soldiers in Aleppo on September 8. 
A Syrian man wounded by shelling sits on a chair outside a closed shop in Aleppo on September 4. 
A woman sits in her wheelchair next to her house, damaged by a Syrian air raid, near Homs on August 26. 
Members of the Free Syrian Army clash with Syrian army soliders in Aleppo's Saif al-Dawla district on August 22. 
A man mourns in front of a field hospital on August 21 in Aleppo. 
Wounded civilians wait in a field hospital after an air strike on August 21 in Aleppo. 
People pray during the funeral of a Free Syrian Army fighter, Amar Ali Amero, on August 21. 
A man cries near the graves of his two children killed during a recent Syrian airstrike in Azaz on August 20. 
A Syrian woman holds her dead baby as she screams upon seeing her husband's body being covered following an airstrike by regime forces on the town of Azaz on August 15. 
A Syrian rebel runs in a street of Selehattin during an attack on the municipal building on July 23. 
Syrian rebels hunt for snipers after attacking the municipality building in the city center of Selehattin on July 23. 
Members of the Free Syrian Army's Mugaweer (commandos) Brigade pay their respects in a cemetery on May 12 in Qusayr. 
Syrian rebels take position near Qusayr on May 10. 
A Free Syrian Army member takes cover in underground caves in Sarmin on April 9. 
Rebels prepare to engage government tanks that advanced into Saraquib on April 9. 
Men say prayers during a ceremony in Binnish on April 9. 
A young boy plays with a toy gun in Binnish on April 9. 
A Free Syrian Army rebel mounts his horse in the Al-Shatouria village near the Turkish border in northwestern Syria on March 16, a year after the uprising began. 
Syrian refugees walk across a field before crossing into Turkey on March 14. 
A rebel takes position in Al-Qsair on January 27. 
A protester in Homs throws a tear gas bomb back towards security forces, on December 27, 2011. 
A man stands under a giant Syrian flag outside the Umayyad Mosque in Damascus on December 24, 2011. 
A member of the Free Syrian Army looks out over a valley in the village of Ain al-Baida on December 15, 2011. 
Members of the Free Syrian Army stand in an valley near the village of Ain al-Baida, close to the Turkish border, on December 15, 2011. 
Displaced Syrian refugees walk through an orchard adjacent to Syria's northern border with Turkey on June 14, 2011, near Khirbet al-Jouz. 
A Syrian man holds up a portrait of President Bashar al-Assad during a rally to show support for the president in Damascus on April 30, 2011. 
Syrians rally to show their support for President Bashar al-Assad in Damascus on April 30, 2011. 
A screen grab from YouTube shows thick smoke rising above as Syrian anti-government protesters demonstrate in Moaret Al-Noman on April 29, 2011. 
A screen grab from YouTube shows Syrian anti-government protesters run for cover from tear gas fired by security forces in Damascus on April 29, 2011, during the "Day of Rage" demonstrations called by activists to put pressure on al-Assad. 
Syrians wave their national flag and hold portraits of al-Assad during a rally to show their support for their leader in Damascus on March 29, 2011. 
A woman sits by the hospital bed of a man allegedly injured when an armed group seized rooftops in Latakia on March 27, 2011, and opened fire at passers-by, citizens and security forces personnel according to official sources. 
Syrian protesters chant slogans in support of al-Assad during a rally in Damascus on March 25, 2011. 
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- Samantha Power's Pulitzer-Prize winning study found U.S. very slow to fight evil regimes
- Peter Bergen: Her work is a perfect description of U.S. dithering on the Syria crisis
- U.S. is likely to strike Syria but lacks a strong basis in international law, he says
- Bergen: U.N. won't support a strike, but NATO and Arab League could back it
Editor's note: Peter Bergen is CNN's national security analyst, a director at the New America Foundation and the author of "Manhunt: The Ten-Year Search for bin Laden -- From 9/11 to Abbottabad."
(CNN) -- What is widely recognized as the most authoritative study of the United States' responses to mass killings around the world -- from the massacres of Armenians by the Turks a century ago, to the Holocaust, to the more recent Serbian atrocities against Bosnian Muslims and the ethnic cleansing of the Tutsis in Rwanda -- concluded that they all shared unfortunate commonalities:
"Despite graphic media coverage, American policymakers, journalists and citizens are extremely slow to muster the imagination needed to reckon with evil. Ahead of the killings, they assume rational actors will not inflict seemingly gratuitous violence. They trust in good-faith negotiations and traditional diplomacy. Once the killings start, they assume that civilians who keep their head down will be left alone. They urge cease-fires and donate humanitarian aid."
This is an almost perfect description of how the United States has acted over the past two years as it has tried to come up with some kind of policy to end the Assad regime's brutal war on its own people in Syria.
The author who wrote the scathingly critical history of how the United States has generally dithered in the face of genocide and mass killings went on to win a 2003 Pulitzer Prize for her book "A Problem from Hell: America and the Age of Genocide."
A decade after winning the Pulitzer, that author is now the U.S. ambassador to the United Nations. Her name, of course, is Samantha Power, and she is a longtime, close aide to President Barack Obama. She started working for Obama when he was a largely unknown junior senator from Illinois.
Power called her 610-page study of genocide "A Problem from Hell" because that's how then-U.S. Secretary of State Warren Christopher referred to the Bosnian civil war and the unpalatable options available to the U.S. in the early 1990s to halt the atrocities by the Serbs.
One of the U.S. officials that Power took to task in her book is Susan Rice who, as the senior State Department official responsible for Africa, did nothing in the face of the genocide unfolding in Rwanda in 1994.
Rice is quoted in the book as suggesting during an interagency conference call that the public use of the word "genocide" to describe what was then going on in Rwanda while doing nothing to prevent it would be unwise and might negatively affect the Democratic Party in upcoming congressional elections.
Rice later told Power she could not recall making this statement but also conceded that if she had made it, the statement was "completely inappropriate, as well as irrelevant."
Rice is now Obama's national security adviser.
In 2012, at Power's urging, Obama announced the creation of an interagency task force to help stamp out atrocities around the world. Called the Atrocities Prevention Board, it was led by Power during its first year. Meanwhile, the body count in Syria kept spiraling upward.
For the past two years, Obama hasn't wanted to intervene militarily in Syria.
Who would? The country is de facto breaking up into jihadist-run "emirates" and Alawite rump states. It is also the scene of a proxy war that pits al Qaeda affiliates backed by Qatar and Saudi Arabia against Hezbollah, backed by Iran.
U.S. military action in Syria expected 
Analysis: Next steps for U.S. in Syria Whoever ultimately prevails in this fight is hardly going to be an ally of the U.S. It's an ungodly mess that makes even Iraq in 2006 look good.
It is, in short, a problem from hell.
Power, Rice and Obama today face some of the very same unpalatable choices that have confronted other U.S. national security officials as they tried to prevent mass killings in other distant, war-torn countries.
They can continue to do little as the Syrian civil war drags on into its third year with 100,000 dead and rising. It's a state of affairs now compounded by the fact that the regime of President Bashar al-Assad appears not only to have crossed the "red line" with its use of chemical weapons but seems to have now sprinted past that line, killing hundreds with neurotoxins in a Damascus suburb, according to U.S. Secretary of State John Kerry. He's blasted those attacks as something that "should shock the conscience of the world. It defies any code of morality."
Doing nothing will not be treated kindly by future historians writing in the same vein as Power.
The issue now in Syria is not simply that al-Assad is massacring his own civilians at an industrial rate, but he is also flagrantly flouting a well-established international norm by this regime's reported large-scale use of neurotoxins as weapons against civilians. It seems inconceivable that the United States as the guarantor of international order would not respond to this in some manner.
But on what authority? There is scant chance of a U.N. resolution authorizing military action. When she was U.N. ambassador, Rice skillfully ushered a resolution through the Security Council that authorized military action in Libya in 2011. But Russia and China will almost certainly veto any similar kind of resolution on Syria.
Russia is one of Syria's few allies, and Russia and China are generally staunchly against any kind of international intervention in the affairs of other countries, no matter how egregious the behavior of those states might be.
That leaves the possibility of some kind of unilateral action by the United States.
The U.S. regularly infringes the sovereignty of countries such as Pakistan and Yemen with CIA drone strikes on the novel legal theory that terrorists planning strikes on the U.S. are living in those nations and those countries are either unable or unwilling to take out the terrorists on their territory -- and therefore their sovereignty can be infringed by drone attacks.
But making a claim that the Syrian regime threatens the U.S. is implausible, and therefore some kind of unilateral American action seems quite unlikely.
In 1986, the Reagan administration launched air strikes at the homes of Libyan dictator Moammar Gadhafi, but only after an incident in which Libyan agents had bombed a disco in Berlin, killing two American servicemen. No such casus belli exists with Syria today.
Since neither a U.N. authorized military mission nor a unilateral American strike seem likely, what options are left?
One appealing option could be something along the lines of the Kosovo model. The Kosovo War in 1999 was entirely an air war in which no American soldiers were killed. The goal of the air campaign was to push Serbian forces out of Kosovo. Russia was allied with the Serbs so, as in the Syrian case today, there was no chance a U.N. resolution authorizing force would pass.
Instead, the war was conducted under the NATO collective security umbrella. Kosovo is, of course, in Europe, and NATO is a Europe-focused security alliance while Syria is the Middle East, so NATO action there would be much more problematic.
(A NATO force does fight in Afghanistan today, but that is only because one of its member states, the United States, was attacked on 9/11 from Afghanistan by al Qaeda, which triggered NATO's Article 5, the right to collective self-defense of the members of the alliance.)
If an air war were to be launched against Syria, one scenario could be that Turkey, a member of NATO, could invoke Article 5 because Syria has fired into its territory on a regular basis.
So far, Turkey has proved reluctant to invoke Article 5 but the reported large-scale use of chemical weapons by the Assad regime might change the calculus of the Turks.
A further source of legitimacy for military action could be some kind of authorization by the Arab League. The Arab League is generally a toothless talking shop, which seemed to have surprised even itself two years back when it endorsed military action against Gadhafi.
That endorsement gave substantial international legitimacy to the subsequent air campaign against Gadhafi, led by the United States and other NATO countries such as France.
It is hard to believe that some kind of military action against Syria won't now take place, likely in the form of U.S. cruise missile attacks from ships in the Mediterranean.
Such attacks have the merit that they won't put U.S. aircraft at risk, which could well encounter problems with Syria's well-regarded air defense systems. And the operation will likely have the blessing of some mix of NATO and Arab League authorizations, giving it at least some semblance of international legitimacy.
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