The Syrian battlefield has been muddied by the presence of several Islamist groups on both sides in the civil war, which erupted in March of 2011.
Thousands of Iranian-backed Hezbollah fighters have joined Mr. Assad.
The rebel group Jabhat al-Nusra, which the U.S. considers to be a terrorist organization, is allied with al Qaeda in Iraq. That alliance is considered one of the more effective rebel groups.
The militants fighting alongside the rebels don’t recognize the authority of the Syrian opposition coalition or Gen. Salim Idriss, the head of the Supreme Military Council of the Free Syrian Army — the coalition’s military wing.
The opposition controls territory mostly in the northern and eastern parts of Syria, and some pockets in the west.
The strength of the Free Syrian Army is not clear. There are also no reliable statistics on the number of militants in the rebels’ ranks.
Despite making gains on the ground, the rebels have been powerless against Mr. Assad’s air force.
“Any Western military operation must focus on destroying the regime’s air force,” Kamal al-Labwani, a member of the Syrian opposition coalition’s defense and security committee, said in a phone interview from Istanbul, Turkey.
A U.S.-led operation would likely be brief, involving cruise missiles and B-2 bombers striking military targets but not chemical weapons facilities, according to multiple sources.
Analysts say a limited strike is unlikely to tip the battle in favor of the rebels.
“If the U.S. decides to use a target set that would significantly degrade the government’s military capability, then it would very much be a game-changer,” said Elizabeth O’Bagy, a senior research analyst at the Institute for the Study of War. “However, the assessment I have been getting is that this will be a punitive attack that is meant to send a message to the regime, and it will not significantly change the regime’s balance of power.”
Barry Pavel, director of the Atlantic Council’s Brent Scowcroft Center on International Security, agreed.
“I would advocate a longer and more intensive air campaign than that which I think we are about to witness,” said Mr. Pavel, a former senior director for defense policy and strategy on the National Security Council staff under President Obama and former President George W. Bush.
The Syrian opposition is counting on an operation that brings about a change of leadership in Damascus.