American-led and Arab-backed air strikes carrying the fight against Islamic State from Iraq into Syria have dragged Washington into a new Middle East war - exactly the kind of conflict Barack Obama spent his presidency trying to avoid.
No one doubts this dramatic escalation presages a long conflict that could spill into neighbouring states and that U.S. air power alone cannot win.
Analysts who have watched Islamic State take parts of Syria and seize territory in Iraq believe it may be possible to contain the group but it will hard to dislodge it.
Washington is clearly bracing for the long haul. The sudden blitz on Tuesday was only the start of a "campaign to degrade and ultimately destroy" Islamic State, said General William Mayville, the Pentagon operations director.
"America can no longer step back from the Syrian conflict", which Obama had avoided even after President Bashar al-Assad last year crossed the president's "red line" by using nerve gas against the rebels, said Fawaz Gerges, Middle East expert at the London School of Economics.
"America's deepening involvement will be with us for the next few years, even after the departure of Barack Obama from the White House," he said.
What America is joining in Syria is a war that has already cost 190,000 lives and forced 10 million from their homes.
Shi'ite Iran and Gulf Sunni states led by Saudi Arabia are backing their sectarian proxies and Syria is a magnet for foreign jihadi fighters, who overwhelmed mainstream Sunni rebels last year and declared an Islamic "caliphate" in June.
BUILDING LOCAL FORCES
Islamic State's ruthless methods - mass executions, massacring civilians and beheading captives - have caused alarm across the world and led to the U.S. military action.
A senior U.S. official said degrading Islamic State "is a necessary condition to getting to the political solution everybody wants to see in Syria".
As long as Islamic State is allowed to control "what is effectively a quasi-state the size of Jordan ... then the chances of political outcomes and de-escalating conflicts are increasingly minimal," the U.S. official said.
He and other allied officials say parallel to the military campaign, the plan is to train and moderate rebel forces to fight Islamic State and deploy in territory that would be vacated by the militants.
While the Obama administration has secured congressional funding for training in Saudi Arabia - around 5,000 more fighters for the Free Syrian Army - diplomats said it would take time for the moderate rebels now fighting both the Assad government and the Islamic State to fill in the vacuum.
Obama delayed moving into Syria against Islamic State, ultra-violent Sunni jihadis spawned by al Qaeda who have hijacked revolts by Syria's Sunni majority against the minority Alawite Assad family rule and by the Sunni minority in Iraq.
Obama waited for Iraqi politicians and their sponsors in Iran to ditch Nuri al-Maliki, the Shi'ite prime minister whose sectarian policies alienated the Sunnis as well as self-governing Kurds in northern Iraq, and replace him with a more inclusive government under Haidar al-Abadi.
Then he assembled a coalition of Sunni Arab partners, including Saudi Arabia, Jordan, Bahrain, the United Arab Emirates and Qatar.
Now that air strikes against Islamic State have begun, the focus is shifting to what will unfold on the ground, where Obama, mindful of America's experience in Iraq and Afghanistan, has pledged not to deploy troops.
Even with a force of 160,000 at the peak of the occupation of Iraq, the United States could not stabilise the country because there was no Iraqi consensus on sharing power and ending sectarian divisions.
In Afghanistan, which US-led NATO forces are due to leave at the end of the year, there is debate about whether air and drone strikes that often caused civilian casualties contained or boosted the Taliban.
Islamic State is clearly savouring the dilemma of its opponents. Its spokesman, Abu Mohammad al-Adnani, taunted America: "If you fight it (Islamic State), it becomes stronger and tougher. If you leave it alone, it grows and expands."
On the ground, Islamic State fighters are embedding themselves in towns they control in Iraq and Syria, such as Mosul and Raqqa, readying for a guerrilla war that will include forays into neighbouring states.
"They would restructure their forces in small groups, and they control major cities ... they have eight million Syrians and Iraqis (as) hostages", Gerges said. The group's attitude is: "You want to come after me, you're going to have to kill a lot of civilians", he said.
With limited capacity to mount spectacular, al Qaeda-style attacks abroad, experts believe Islamic State will attack the soft underbelly of the west and allies in Lebanon, Turkey, Jordan and the Gulf, including their nationals and diplomats.
"All the five countries that attacked (Islamic State), Bahrain, Qatar, Saudi Arabia, Jordan and the UAE, are targets", said Jamal Khashoggi, a leading Saudi commentator and Editor-in-Chief of Al Arab News Channel.
The backlash has already started. Algerian militants have beheaded a French hostage to punish Paris for joining air strikes against Islamic State.
'THEY DON'T WANT ANOTHER IRAQ'
U.S. strategy, analysts and officials say, is to encourage Sunni forces to turn on Islamic State, by offering a share in national power as well as local control over their own affairs - a strategy that looks clearer in Iraq than it does in Syria.
In Iraq, the aim is to re-enlist the Sunni tribes that ousted al Qaeda in the Sunni Awakening of 2006-08.
Alienated by the Maliki government, many tribal fighters have made common cause with Islamic State. The idea is to give Sunni forces local control, but linked to a national guard that will also absorb Shi'ite militias that police their own areas.
"What the national guard does is basically give a promise to the people in these provinces that if you stand up as part of the process of securing your own population, your own families, your own communities, you will be taken care of in terms of salaries and pensions and being able to have a sustainable life and future for your families," said a U.S. official in Iraq.
In Syria, the task is more complicated because of Western opposition to the Assad government.
Fighting Islamic State would be easier if Assad were removed, paving the way for coordination between a government under new leadership and mainstream rebels, said an official from a NATO member of the coalition against IS.
"Everybody, including the Americans, knows that to make headway against Islamic State you need Assad to go, just as you needed Maliki to go in Iraq," the official said.
This would require the removal of a "smallish clique" around the president, and those responsible for the worst atrocities - leaving in place institutions including the bulk of the army as foundations for a future transition.
For a post-Assad transition to happen, moderate rebels must recapture momentum on the ground. A U.S. State Department official stressed the need to strengthen those rebels and said the U.S. was not going after Assad for now "because the moderate opposition isn't ready yet".
But above all detente between the two rival powers in the Middle East - Saudi Arabia and Assad's backer Iran, and a bargain between Tehran and Washington, would set the stage for a political deal.
"If I am Assad, I will be worried. There is a consensus of not wanting to do business with Assad, that this is a losing battle," a Western diplomat said.