—SYRIAN REGIME: Despite major defections and the loss of significant territory to rebels, the Syrian military remains a potent force against a poorly armed opposition. President Bashar Assad's inner circle has largely remained cohesive and united, avoiding high-level defections that sapped the strength of other regimes, such as Moammar Gadhafi's in Libya, during Arab Spring uprisings. Assad's closest advisers include his younger brother, Maher, who commands forces protecting the capital, as well as the heads of four intelligence agencies that are playing key roles in the government's crackdown.
—PRO-REGIME MILITIAMEN: Shadowy fighters, known as shabiha, recruited from the ruling elite's Alawite sect operate as hired muscle for the Syrian regime. They are believed to be carrying out some of the most brutal attacks of the conflict, allowing Assad's government to deny direct responsibility for them.
—SUPREME MILITARY COUNCIL: Syria's main rebel units, known together as the Free Syrian Army, regrouped in December under a unified rebel command called the Supreme Military Council, following promises of more military assistance once a central council was in place. The Western-backed council is headed by Gen. Salim Idriss, who defected from the Syrian army, and a 30-member group of senior officers. Idriss spent 35 years in the Syrian military and is seen as a secular-minded moderate. The non-lethal aid the U.S. pledged Thursday in Rome will be directed to the military council.
—LOCAL BRIGADES AND MILITARY COUNCILS: Local units made up of tens of thousands of autonomous rebel fighters have very little, if any, central organization or command structure. Many of the fighters defected from the military; others are Syrian citizens who took up arms against the regime.
—JABHAT AL-NUSRA: An Islamist extremist group that has been behind some of the rebels' most significant battlefield successes. The U.S. has designated al-Nusra a terrorist organization, saying it is affiliated with the al-Qaida network. Al-Nusra has claimed responsibility for most of the deadliest suicide bombings targeting regime and military facilities. The presence of Islamic extremists among the rebels is one reason the West has not equipped the Syrian opposition with sophisticated weapons, such as anti-aircraft missiles. Al-Nusra has gained popularity among some rebels for its effectiveness while alienating other, more secular-minded fighters.
—FOREIGN FIGHTERS: Syria has become a magnet for foreign fighters and jihadists who also flocked to the conflicts in Iraq and Afghanistan. No credible count of them exists, but anecdotal evidence suggests fighters from Libya, Yemen, Tunisia, the Netherlands and Britain are fighting against Assad's regime. Rebel commanders downplay the presence of foreign fighters, saying the conflict is purely a Syrian uprising.