As President Obama considers airstrikes in Syria and calls for an international coalition to fight the Islamic State, analysts say no country in the area other than Syria would be a likely candidate to send needed combat troops against the militants.
"You cannot defeat terrorists or an insurgency unless you've got boots on the ground," said Chris Harmer, a former U.S. military strategist now at the Institute for the Study of War.
In the last 18 months, Islamic State militants have battled Assad's military, Hezbollah and Iranian Revolutionary Guards Corps, the al-Qaeda affiliate Jabhat al Nusra, Iraqi security forces and the Kurdish peshmerga — and "they defeated all of them," Harmer said.
"The idea we're going to destroy these guys with air power (alone) is a non-starter," he said. "You have to have somebody go head-to-head with (Islamic State) and defeat them. You have to have a local ally to go in with you."
A year after Obama almost launched airstrikes against's Syria, the United States is now attacking the regime's most powerful enemy — Islamic State militants who control territory spanning Syria and Iraq. And nearly three weeks after the U.S. began targeted airstrikes in Iraq against the Islamic State, Obama is weighing whether to move those tactics into Syria but insists there will be no coordinating with President Bashar Assad, who has slaughtered more than 200,000 of his people.
In a news conference Thursday, Obama said defeating Islamic State militants in Syria does not mean siding with Assad.
"We will continue to support a moderate opposition in Syria to give people a choice other than ISIL (Islamic State) and Assad," Obama said. But he said the plans are still in the works. "We don't have a strategy yet."
Shadi Hamid, an analyst with the Brookings Institution, said the only viable alternative is to partner with Syrian opposition groups. But lacking significant Western support, their numbers have dwindled and they are currently disorganized and at risk of losing their last major stronghold in the city of Aleppo, where they are sandwiched between offensives by the Syrian government forces and the Islamic State.
"If we don't do anything to boost the rebels, then any airstrikes against (the Islamic State) will benefit the Assad regime," Hamid said. "We may not like that, but that is the inevitable result."
Ryan Crocker, a former ambassador to Syria under President Clinton and to Iraq under President George W. Bush, pointed out that Iraq regained control of the Mosul Dam this month from Islamic State militants with help from ground troops, bolstered by U.S. airstrikes. He said ground forces from somewhere also will be needed in Syria.
"The question is who?" Crocker said. "There's a reasonably good chance Assad will fill the void if the Free Syrian Army doesn't."
The Islamic State earlier this week took the Syrian government's Tabqa air base, and Syrian airstrikes have targeted the militant group in eastern Syria. But until recently, the two groups have had a defacto alliance. The Islamic State makes money through oil sales to Assad from wells in territory the group controls, and the two share a common enemy: the other rebel forces, Hamid said.
Syrian Foreign Minister Walid al-Moallem said this week that Syria was willing to participate in a coalition including the U.S. and Britain, but any violation of Syrian airspace would be considered "an act of aggression."
World powers "will soon have to choose what is more important: a (Syrian) regime change to satisfy personal antipathies … or finding pragmatic ways to unite efforts against the common threat," Moallem said, according to Al Arabiya News.
Syria would welcome U.S. involvement, because that would create a perception of collaboration with the Assad regime, former State Department official Fred Hof wrote this week in The New Republic.
That perception, though, could drive a wedge between the U.S. and its regional partners — Saudi Arabia, United Arab Emirates and Qatar, which bankroll the Syrian opposition with hopes of toppling Assad, and which regard Iran, Syria's main ally, as their chief rival and threat in the region.
Such a rift could help Assad stay in power and strengthen Iran, wrote Hof, a resident fellow at that Atlantic Council's Rafik Hariri Center for the Middle East.
Edward Djerejian, former U.S. ambassador to Syria, said the United States should build up moderate elements in the Syrian opposition to enable them to take over the area that the Islamic State now controls.
"No outside country, not the Turks, not Jordan, not Iraq," is likely to send troops, "and the FSA (Free Syrian Army) isn't powerful enough to do it," Djerejian said. Unless the moderate Syrian opposition is built up with American assistance, "it ends up being the Syrians," he said.