The United States has yet to explain exactly what victory might look like in its war against the Islamic State group, but it is becoming clear that success will depend heavily on political events in Syria and Iraq that are beyond its control, experts say.

The US strategy to "destroy" the IS jihadists is predicated on a series of high-stakes gambles that could take years to play out, particularly in Syria, where Washington is betting it can forge a dominant "moderate" rebel force.

The US administration is "recognizing that it is going to take a long time even in the best case scenario," Karl Mueller, senior political scientist at the RAND Corporation think tank, told AFP.

- Long haul looms -

Despite dramatic images of fighter jets bombing jihadist targets, President Barack Obama and his commanders have warned repeatedly that Americans should brace themselves for a years-long struggle and that the initial air raids will not produce miracles.

"I think this is going to be a generational challenge," Obama said this week.

The president and his deputies hope the US-led air strikes in Iraq and Syria will serve as a firewall against the rampaging Sunni militants, buying time to build up local forces and fuel political momentum against the group.

"What our military operations can do is to just check and roll back these networks as they appear and make sure that the time and space is provided for a new way of doing things to begin to take root," Obama said.

Based on rough outlines offered by US officials, the war strategy is counting on defeating IS fighters first in Iraq through a combination of Kurdish forces, Iraqi army troops, Shiite volunteers and a militia or "national guard" of Sunni tribes -- which does not yet exist.

And in Syria, Washington is pinning its hopes on training and arming a new rebel army, at a rate of about 5,000 fighters a year.

At that pace, it will take about three years before the force is big enough to prevail against the IS group, according to the military's top officer, General Martin Dempsey.

If the American-trained rebel force could push back the IS group, it might eventually topple the Damascus regime, but officials have not articulated precisely what they have in mind in Syria.

-- Hoping for change in Iraq --

In Iraq, rolling back IS extremists will hinge not on weapons or tactics but on the Shiite-led Baghdad government giving up its sectarian ways and reaching out to the country's alienated Sunni community, analysts said.

Much is riding on Iraq's new Prime Minister Haider al-Abadi, who has yet to signal a dramatic change, said Marina Ottoway, a senior scholar at the Woodrow Wilson Center.

"So far, the government has not taken any concrete decisions that could convince Sunnis and Kurds that their interests are now protected," Ottoway wrote in a new paper.

After the 2003 US invasion of Iraq, general David Petraeus, who later became commander there, famously quipped -- "Tell me how this ends."

The plan to vanquish the IS militants at times appears uncertain about "how this ends," but that is partly because the Americans had to act quickly to stop a lightning advance by the jihadists, said Mueller.

"The near term objective is to hold their (IS) advances and prevent the situation from getting worse," he said.

For the longer-term goal, the administration's strategy has an improvisational aspect, he said. "They are, to some extent, playing it by ear."

Despite a myriad of uncertainties, the IS group is not invincible and could be pushed into obscurity if it faced sustained pressure, particularly from Syrians and Iraqis opposed to its brutal ways, some experts said.

"If ISIL (IS) suffers big defeats in Iraq in the coming year, as I expect, their strength in Syria may suffer too," said Michael O'Hanlon of the Brookings Institution.

"Just because we can't clearly see an end state shouldn't preclude developing some allies and some leverage," said O'Hanlon, who has criticized Obama for not taking action sooner.

The outcome of the campaign will likely come down to politics in Iraq, Syria and among its neighbors, said Ottoway.

If the Iraqi government and Syrian opposition leaders fail to rise to the moment and shed their sectarian, ideological agendas, the US intervention will -- at best -- only slow the IS group's advance, she said.

As with US action in Iraq and Afghanistan over the past decade, she said, "a successful military intervention will be undermined by the hollowness of the political strategy."