The United States Monday built a humanitarian and legal case for military action against Syria, rooted in the proposition that an "undeniable" chemical attack had shattered international codes of war.
US rhetoric, led by an emotional indictment of Syria by Secretary of State John Kerry, is suddenly hawkish: a remarkable turn, since the White House has spent months trying to halt a slide into another Middle Eastern war.
Officials cautioned that no final decisions on force nor a timeline for action had been made.
There was however a growing sense in Washington that the clock was relentlessly ticking down towards US strikes against President Bashar al-Assad's regime: the only questions were when and how.
The shift in tone has been swift.
Late last week, President Barack Obama was warning about the danger of new entanglements in a blood-soaked region, which may not "turn out well and get us mired in very difficult situations."
But it was clear a combination of what Kerry called "gut-wrenching" footage of dying children in a Damascus suburb last week, and what officials see as solid intelligence of regime culpability, shifted the US position over the weekend.
The administration made a dual case: that the use of such heinous arms against civilians, regarded by the world as taboo for decades, must not stand. Also, they argue, US national interests are now at stake.
Kerry -- perhaps eyeing Russia, the Syrian ally which has warned the West not to make a "grave mistake" -- said "common humanity" dictates the need to ensure the attack last week is not repeated.
And with polls showing antipathy among Americans for another Middle East misadventure, the White House began to make a domestic political argument.
"The use of these weapons on a mass scale, and the potential risk of proliferation, is a threat to our national interests," said White House spokesman Jay Carney.
Any final case for military action would likely be made by Obama himself to the American people, in a national address, for which Kerry's remarks at a State Department press conference clearly laid the groundwork.
It was unclear exactly which international law statutes the administration will use to build its case, but the 1925 Geneva Protocol -- though never fully ratified -- provides a codified framework outlawing the use of poison gases in war.
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The administration has also made clear that it will not go it alone.
While a UN Security Council resolution authorizing force would likely draw a Russian veto, the precedent for action by an international coalition without such a mandate was set by the 1990s Kosovo conflict.
Any US military action in Syria would likely be constrained in scope -- likely to start with cruise missile strikes launched from US, and possibly allied, ships and submarines.
Analysts say possible targets could include military units implicated in the attack last week, which opposition forces say killed up to 1,300 people.
Any strike must be sufficiently punitive to deter further use by the Assad regime of chemical weapons.
But there is no appetite in Washington for prolonged involvement -- the mantra is "no boots on the ground" and senior officials say the notion of a "no-fly" zone in Syria is not on the table.
Stiffened US rhetoric appears to offer Obama little wiggle room. The same is true of his warning a year ago that the use of chemical weapons would cross a US "red line" -- comments which placed presidential credibility on the line.
"I think a response is imminent," Republican Senator Bob Corker said Monday.
"I think you are going to see a surgical, proportional strike against the Assad regime for what they have done and I support that," Corker said on MSNBC.
Republican House Armed Services committee chairman Howard "Buck" McKeon agreed, saying there can be no impunity for the use of chemical weapons.
"The president cannot fail to act decisively," he said.
Obama has spent months trying to avoid being sucked into a war which has killed at least 100,000 people, after extracting US troops from Iraq, and as he brings them home from Afghanistan.
His defenders point out he is hardly a reluctant commander-in-chief: He leads a ruthless drone war worldwide and risked his presidency to kill Osama bin Laden.
But his instincts are to avoid new foreign quagmires and he built his political career on raging against "dumb wars."
A Syrian campaign would also threaten Obama's chosen legacy -- one of ending wars, not of opening new fronts.