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.
IS THIS A SECTARIAN CONFLICT?
The Syria conflict is whipping up sectarian fervor. Sunni-Shiite tensions in the region have risen sharply, particularly since Hezbollah raised its profile by fighting in Qusair. Sunni hard-liners view Hezbollah's intervention as a declaration of war by Shiites against Sunnis, and have called on Sunnis to fight in Syria. This could increase the flow of foreign militants into Syria. Already several thousand foreign militants are believed to be fighting among the rebels.
COULD THIS RAISE EAST-WEST TENSIONS?
Russia has been a major weapons supplier to the Syrian regime. Russia said repeatedly it would honor its contracts to deliver advanced missiles to Syria, including S-300 air defense systems, ignoring appeals by the West to halt shipments.
Russian officials played down the threat of an arms race Friday. Asked if Russia could retaliate for the U.S. decision to arm the rebels by sending the S-300s, President Vladimir Putin's foreign affairs adviser, Yuri Ushakov, said the two sides aren't competing in Syria.
The rebels, meanwhile, could obtain weapons from other Western sources. Last month, the European Union decided to let its arms embargo against Syria lapse, enabling individual members to arm the rebels. Britain and France had pushed for the measure, though they said at the time such shipments were not imminent.
WHAT ABOUT CHEMICAL WEAPONS?
The regime's chemical weapons stockpiles are a major wild card in the conflict.
The Obama administration says the regime carried out multiple small-scale attacks with such weapons, killing up to 150 people. The findings announced Thursday were aided by evidence sent to the United States by France, which, along with Britain, has said it had determined that Assad's government used chemical weapons.
Experts say Assad might have been trying to terrorize rebels and civilians, while not causing mass casualties that would trigger greater Western military involvement.
The regime is believed to be in control of its stockpiles for now. Israel has said it would strike to prevent chemical weapons from reaching Hezbollah which has fought with Israel in the past.
WHAT IS HAPPENING TO THE REGION?
The fighting repeatedly has spilled into neighboring Lebanon, Turkey, Iraq, Jordan and the Israeli-controlled Golan Heights, raising fears of a regional conflagration.
Lebanon, still scarred by its own 15-year civil war that ended in 1990, is increasingly on edge. Hezbollah's involvement in Syria has prompted retaliatory rocket fire by Syrian rebels on Hezbollah strongholds in Lebanon.
Israeli warplanes have struck three times at suspected Hezbollah-bound weapons shipments in Syria, and Israeli officials threatened more strikes in the event of future arms deliveries. Assad did not retaliate up to now, but said he would deliver a strategic blow if the Israelis attack again.
The conflict already has fueled a spike in sectarian warfare in Iraq as the Shiite-led government struggles to contain its worst eruption of violence in years amid a wave of Sunni unrest. Syrians have been killed in Iraq and combat-hardened Iraqi fighters have been crisscrossing the frontier.
Turkey has repeatedly struck back at the Syrian military in response to shelling and mortar rounds that landed on its territory. NATO has sent anti-aircraft batteries to Turkey's border area with Syria. In May, two car bombs blamed on Syria killed more than 50 people in a Turkish border town.