A year ago, President Barack Obama was set to bomb Syria -- but balked at the last minute after sizing up beckoning political isolation in his war-weary nation.

Now, he is stepping to the brink again -- but the political winds may have changed.

The execution of captive US journalist James Foley and the fear an ISIL caliphate could become a terror haven have challenged an administration foreign policy built on the conceit that the "tide of war is receding."

ISIL has also set off a national security debate in the nascent 2016 presidential campaign, which will culminate only when the next US leader decides how he, or she, will wield US might abroad.

Antipathy

Obama has made no effort to hide his antipathy to open-ended engagements in the Middle East, despite potent but limited campaigns against Al-Qaeda in Pakistan and in Libya.

But Foley's horrific death was a direct challenge to the Obama government -- and for political reasons alone, it is unthinkable he will not respond.

The prospect that Western ISIL recruits could bring terror to America simply by buying a plane ticket also lands Obama with a new war abroad that he did not seek but may have to wage.

But a serious attempt to crush ISIL, across the dissolved borders of Iraq and Syria, threatens to evolve into the kind of open-ended commitment in the Middle East that Obama's presidency was built on avoiding.

Nevertheless, the Pentagon is preparing options for possible US action on the group's strongholds in Syria.

Obama has long resisted the temptation to get sucked into the vicious civil war, and last year ordered a last-minute halt to US air strikes to punish the alleged use of chemical weapons.

He also set a narrow mandate for new US air strikes targeting ISIL in Iraq —- to protect American diplomats and prevent a genocide of ethnic Izadis.

Yet the killing of Foley, highlighting a direct threat to Americans, may make military action in Syria an easier sell this time around.

Tipping point

Stiffened administration rhetoric has heightened expectations Obama could soon launch a new Syrian front against a group he last week branded a "cancer."

A top advisor, Ben Rhodes, called Foley’s execution a terrorist attack, warning, "if you come after Americans, we're going to come after you."

A masked ISIL militant speaks next to US journalist James Foley at an unknown location in this still image from an undated video posted on a social media website.

Defense Secretary Chuck Hagel warned IS was "beyond" any terror group Washington had seen and the top military officer Martin Dempsey said the United States could only defeat the group by hitting it in Syria.

A former senior administration official said the rhetorical shift may signal a White House "tipping point".

"It does seem to me, they have stepped up a gear —- from second, to fourth, on how to deal with (ISIL)."

Drums of war are beating anew on Capitol Hill.

"This administration has thus far only dealt with containment," Michael McCaul, chairman of the House Homeland Security Committee told ABC News.

"We need to expand these air strikes so that we can ultimately defeat and eliminate (ISIL)."

White House spokesman Josh Earnest said Obama had so far made no decision on whether to turn US fire on ISIL strongholds in Syria.

But Hussein Ibish, senior fellow with the American Task Force on Palestine, a close observer of US Middle East policy, said Washington had little choice.

"The US is in fact (at) war with (ISIL). It's going to escalate, there's no way out. It cannot but escalate," he said.

The sudden potency of ISIL is also grading the emerging terrain of the 2016 presidential race.

US elections usually turn on domestic issues, but given global turmoil, a row over America's place in the world is certain to rage through the campaign to find Obama's successor.

Democrat Hillary Clinton noted that Obama had not taken her advice to arm rebels in Syria.

In an interview with The Atlantic magazine the former secretary of state appeared to advocate an "overarching framework" of US foreign policy based partly on combating extremism in the Middle East modeled on the US struggle against communism.

Obama has by contrast resisted foreign policy doctrines, and shied away from the "war on terror" framing of his predecessor George W. Bush.

ISIL has also complicated life for Senator Rand Paul, the libertarian Republican, who warned in June that US meddling had already created a "jihadist wonderland".

Paul warned Sunday that "gung ho" Clinton, a potential general election foe, could start a new Middle East war.

Other Republicans blame Obama's supposed weak leadership for the chaos in the Middle East.

But none has gone as far as Rick Perry, seeking redemption after the mockery of his 2012 run. The Republican governor of Texas suggested Washington should be ready to send combat troops back to Iraq to fight IS.

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