A few days ago it appeared that a US missile strike would have broad Congressional support, a moral justification (if not a legal basis) and the implicit support of much of the world.
No longer. As one American blogger put it: "This is a war with no clear objective, thus no strategy to attain it, no legal basis and no public support."
It is certainly true that Americans have little appetite to get embroiled in another Middle East conflict.
Later today, the CIA will declassify the intelligence that links the Syrian regime to the chemical attack on August 21. It had better be strong, given the fiasco of faulty intelligence in 2003 in the run-up to the Iraq War.
A striking number of armchair generals and TV pundits are warning of the danger of unintended consequences; that a punitive attack will suck America into the civil war. It's the old Colin Powell rule: If you break it, you own it.
America emphatically does not want to own Syria.
And after a flurry of ineffective diplomacy at the UN yesterday, now there's the tricky legal question. Precisely on what basis is America acting?
The UN Secretary General himself has asked America to give the weapons inspectors time to finish their work.
But as the time grows between the August 21 chemical weapons attack and a US response, so the image of dead children will fade. And Americans will have even less tolerance for military engagement.
The choreography of an attack has been complicated further by Cameron's troubles in Parliament. Sources here are saying that America won't wait for Britain.
So Obama may act virtually alone, against US public opinion, without UN authorisation, with dubious legality and in the face of multiple questions about his strategy.
Some in Congress are waking up, accusing the President of rushing into another war. Others are saying the opposite: he's displaying chronic indecision and jeopardising American credibility and global leadership.
What seemed so simple - the punishment of a dictator who it seems has committed one of the most ruthless crimes of the 21st Century - suddenly seems very complicated.
It's an unexpected crisis for the White House as Barack Obama struggles with one of the most fateful decisions of his Presidency.