Two weeks into Russia's air war in Syria, some observers say the narrow scope of the US-led mission in Syria -- to conduct drone and plane strikes against Islamic State jihadists -- needs an urgent rethink.
The Pentagon insists Moscow's bombing campaign in support of President Bashar al-Assad will not alter the US-coalition's own military mission in the devastated nation.
But the stubborn fact remains: Russia's presence has forced the US military to adapt to a suddenly much more complex battle space.
Patrick Skinner, director of special projects for The Soufan Group intelligence consultancy, said it is a game-changer.
"Things are coming into focus and people are choosing sides in a way that they haven't had to before," he said.
President Barack Obama has until now adopted a cautious line on Syria, bombing IS jihadists and calling for Assad's departure but taking no official part in the country's four-and-a-half-year civil war.
"We've been in a 'doing-just-enough-not-to-lose' effort," said Skinner. "(Russia) is going to snap us out of that yearlong slumber where we thought the biggest issue in Syria was ISIS, where no one else thinks that."
While Moscow says its campaign targets IS, Washington and its allies accuse it of targeting moderate Western-backed rebels with the aim of propping up Assad, a longtime Russian ally.
"This is a civil war -- pro-Assad or anti-Assad; there's not much room for an anti-ISIS focus," Skinner added, using an alternate acronym for IS.
Obama is deeply wary of being drawn into another Middle Eastern quagmire after Iraq, where a US-led coalition toppled Saddam Hussein in 2003 only to see the country explode in sectarian violence.
But now calls are mounting at home for Washington to get its hands dirty, as the death toll from the Syrian war reaches 250,000 and millions of refugees flood onto the roads.
Republican Senator John McCain, a longtime critic of Obama's handling of Syria, called this week for America to scale up its involvement, establishing safe zones for Syrians and moderate Assad opponents -- and even envisaged sending in troops.
"These enclaves must be protected with greater American and coalition air power and likely foreign troops on the ground," McCain wrote in an opinion piece this week. "We should not rule out that US forces could play a limited role in this ground contingent."
White House hopeful Hillary Clinton echoed his call for a no-fly zone during the first Democratic presidential debate this week.
Part of the reason for Washington's reluctance in Syria, argues Joshua Landis, director of the Center for Middle East Studies at the University of Oklahoma, is that it has no credible alternative to Assad -- or IS -- to champion.
"The trouble is that America doesn't have a partner," he said.
A video grab made on October 14, 2015 from a footage on the Russian Defence Ministry's website purporting to show explosions after airstrikes carried out by Russian forces in the Syrian province of Idlib.
So far the Obama administration has shown no sign of an official change in policy, with Defense Secretary Ashton Carter reiterating his stay-the-course approach on Wednesday.
"We will continue to prosecute the counter-ISIL campaign with the same determination and in the same battle space as we have since it started in Syria," he said.
But on the ground, the operational situation is looking ever more challenging.
Obama has insisted he would not let Syria become a "proxy war" with Russia, but some observers suggest it may already be too late.
That is because of the second -- unofficial -- arm in Obama's Syria strategy: the training and arming of moderate Syrian rebels to fight regime forces.
In what Skinner described as America's "most well-known covert action program," the Central Intelligence Agency has been arming and training thousands of anti-Assad fighters.
The rebels have used TOW (Tube-launched, Optically tracked, Wire-guided) missiles, first produced by the United States in the 1970s, to great effect against Assad's Russian-made tanks.
Provided by the CIA and Saudi Arabia, rebels have been hailing the missiles as a game-changer akin to the anti-aircraft Stinger missiles used by the mujaheddin against the Soviets in Afghanistan in the 1980s.
The United States has not officially acknowledged helping these rebels, and it is unclear what -- if any -- military support it will provide now Russia is involved.
US President Barack Obama has insisted he would not let Syria become a "proxy war" with Russia, but some observers suggest it may already be too late.
But the notion of US-built rockets destroying Russian-built tanks prompted observers to suggest America may have sleepwalked into a proxy war with Russia.
"It's a proxy war by happenstance," Jeff White of the Washington Institute for Near East Policy told the Washington Post.
"The rebels happen to have a lot of TOWs in their inventory. The regime happened to attack them with Russian support."
In addition to covert ops, the Pentagon is openly equipping anti-IS Syrian rebels near Raqqa -- creating some risk of Russian bombers killing US-backed rebels, although officials say the area has seen scant attention from Moscow.
Most critically, though, coalition and Russian pilots are now operating in the same space -- and not necessarily with the same objectives.
At least once, coalition pilots scrapped a bombing run because of Russian air activity. And last week, Moscow fired cruise missiles into Syria from the Caspian Sea.
Russia and the United States said Wednesday that they were close to agreeing a deal to avoid clashes between their warplanes over Syria.
"It didn't make us excited they were firing cruise missiles in air space we were operating in," said one defense official, on condition of anonymity.