President Barack Obama's decision to begin arming Syria's rebels deepens U.S. involvement in a regional proxy war that is increasingly being fought along sectarian lines, pitting Sunni against Shiite Muslims, and threatening the stability of Syria's neighbors.
Arming the rebels is bound to heighten U.S. tensions with Russia, a staunch ally of Syrian President Bashar Assad. It could further escalate a brutal, if deadlocked, civil war that has killed nearly 93,000 people and displaced millions, with no end in sight. There are fears that Assad's stockpile of chemical weapons, believed to be one of the world's largest, could fall into the hands of Islamic extremist groups or that he might unleash them if he feels cornered.
Obama's decision marks a turning point for the U.S., which up to now had avoided getting drawn into the conflict militarily. A key U.S. concern had been that U.S.-supplied weapons could fall into the hands of al-Qaida-linked militants fighting alongside the rebels.
However, U.S. credibility was on the line after the White House said Thursday that it has conclusive evidence that Assad used chemical weapons against rebel fighters. Obama has said in the past such use would cross a red line, suggesting greater U.S. intervention.
Washington's decision comes at a time of several military setbacks for the rebels and the growing involvement of Lebanon's Hezbollah militia, which is fighting alongside the regime. Hezbollah's role was key in the capture of the strategic rebel-held town of Qusair earlier this month.
WHAT WOULD THE REBELS RECEIVE?
The full scope of the assistance authorized by the White House is still unclear. But the administration could give the rebels a range of weapons, including small arms, assault rifles, shoulder-fired rocket-propelled grenades and other anti-tank missiles. Rebel commanders say they need anti-tank and anti-aircraft missiles to counter the regime's superior firepower, delivered from warplanes and armored vehicles. But Obama's opposition to sending American troops into Syria and concern about high-powered weapons ending up in the hands of terrorist groups makes it less likely the U.S. will provide sophisticated arms that would require large-scale training.
WHO IS FIGHTING?
The regional context for the Syria conflict is the struggle for influence between Shiite Iran on the one hand and major Sunni power Saudi Arabia on the other, backed by smaller Gulf Arab states, such as Qatar, and non-Arab Turkey.
Assad is part of the Iranian camp, along with Hezbollah. At home, he draws his support largely from Syria's minorities, including fellow Alawites, followers of an offshoot of Shiite Islam, as well as Christians and Shiites. His other foreign backers include Russia and China.
Most rebels are Sunnis. The West, including the U.S., has so far backed the political opposition and provided humanitarian and non-lethal support to the rebels.
WHO HAS THE UPPER HAND?
Hit by defections, regime forces have been stretched thin, a key reason why Assad lost control over large stretches of northern and eastern Syria early in the fighting. However, he has been able to hang on to the capital, Damascus, and other cities, especially in the heavily populated west of the country. Building on the successful capture of Qusair, Hezbollah-backed regime fighters have scored a number of military successes in recent weeks. Pro-Assad troops are now trying to dislodge rebels from the cities of Homs and Aleppo, Syria's largest. The rebels hope the U.S. weapons will give them new momentum.
WHEN WILL IT END?
Neither side has been able to deliver a decisive blow since the uprising against Assad began in March 2011 and escalated into a civil war. Fighting could drag on for months or years.
With Russia and Iran standing by Assad, he seems poised to cling to power for now, even if unable to retake all of Syria. Some predict an eventual division of Syria into regime- and rebel-held areas, with conflict simmering for years.
A fall of the regime, a prospect that appears distant at the moment, would not ensure an end to the fighting. Assad's die-hard supporters might not lay down arms and the rebels are divided between Western-backed moderates, fundamentalist Salafis and al-Qaida loyalists who could battle for control after a collapse of the regime.
Still, a defeat of the regime could curb Iran's influence in the Arab world, weaken Hezbollah in Lebanon and strengthen minority Sunnis in Shiite-dominated Lebanon and Iraq. In one reconfiguration of regional alliances, the Palestinian militant group Hamas last year broke away from Iran's camp over Assad's crackdown on the rebels, fellow Sunnis.