The White House announced Thursday that Syrian President Bashar Assad had crossed a "red line" by using chemical weapons against the opposition. The announcement comes amid calls for greater U.S. engagement in the conflict. We take a look at what is happening in Syria and who the major players are.

Where Do Things Stand In Syria?

The U.N. says at least 93,000 people have died in the fighting in Syria since March 2011. The conflict pits the government of President Bashar Assad against a coalition of rebel forces. The rebels appeared to have the upper hand in the early stages of the fighting, but, as NPR's Deborah Amos reported, after more than a year of a military stalemate, Assad appears to be gaining ground. Just last week, Assad's forces captured Qusair, a key city near the Lebanese border, that had been under rebel control for more than a year.

Who Supports The Sides?

The Syrian regime is backed by Iran and Russia, which is a permanent, veto-wielding member of the U.N. Security Council. Assad is also supported by fighters from Hezbollah, the Shiite militant group from Lebanon. The rebels, on the other hand, are backed by Saudi Arabia and Qatar, as well as Britain and France. President Obama has said Assad must go, but until Thursday's announcement the U.S. approach to the conflict has been cautious.

What Sort Of Weapons Are Being Used?

Iran is supplying weapons to Assad's regime, and Russia has said it will deliver anti-aircraft systems to the country. The Russian move came in response to the European Union's decision to end its embargo on arming the rebels. And, of course, there is now the confirmation by the White House that Assad has used chemical weapons. Meanwhile, as NPR's Kelly McEvers reported in March, "Croatian anti-tank guns and rockets are being sent to the rebels ... with the tacit support of the United States via Jordan [and are] being purchased by Saudi Arabia."

What Are The Regional Effects?

The conflict has created more than 1 million refugees, according to U.N. estimates. And while the U.N. refugee agency determines how to resettle some of them, countries in the region are bearing the brunt. Jordan is facing a refugee crisis, as are Lebanon, a country in which Syria was the main power broker, and Turkey.

But there is another threat: that the Syrian conflict could spread beyond the country's borders, destabilizing the entire region. Indeed, Robert Malley, the program director for Middle East and North Africa for the International Crisis Group, told WHYY's Fresh Air: "This has become not just a war within Syria. It has become a regional, sectarian civil war." The conflict is fast becoming a Sunni-Shiite conflict, drawing in fighters from Lebanon, Iraq and elsewhere.

What's Next?

The White House on Thursday's said it had "high confidence" that Assad's regime was using chemical weapons, and a senior administration official said the U.S. is prepared to offer the opposition military assistance. But speaking on the Senate floor, Arizona Sen. John McCain said he believes military support "is not enough." He, and others, have called for a no-fly zone over Syria and for airstrikes. The White House announcement Thursday comes as the U.S. and Russia are trying to get the Syrian government and the opposition into peace negotiations.

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