Yesterday it became clear that this was no idle threat. US Defence Secretary Chuck Hagel's officials are investigating the gas attack in eastern Damascus which might have killed as many as 1300 people. Hagel said if the Syrian government was found to be responsible, he would provide the President "with options for all contingencies, and that requires positioning our forces [and] positioning our assets to be able to carry out whatever options the president might choose".
In pursuit of that aim he ordered a fourth missile-carrying warship into the eastern Mediterranean, paving the way for a cruise missile attack on Syrian military targets.
However, Pentagon officials were quick to point out that nothing had been decided and that no orders had yet been issued for a military intervention.
And despite the sudden flurry of sabre-rattling it is difficult to avoid the impression that, when faced by an apparent atrocity in a stubborn dictatorship, the US and its main allies have been plunged into paralysis. Only the French have insisted that something has to be done - on Friday, foreign minister Laurent Fabius urged that there had to be "a reaction of force" - but most diplomats are agreed there is unlikely to be a united response from the international community unless the US takes a lead.
That still seems to be a long shot. General Martin Dempsey, the chair of the Joint Chiefs of Staff, claims any military involvement in the country would be counter-productive. In an official letter issued to Democrats on the House Foreign Affairs Committee, he admitted that while the US could destroy Syria's air force it would be an unacceptable risk to use US ground forces.
"Syria today is not about choosing between two sides but rather about choosing one among many sides," wrote Dempsey. "It is my belief that the side we choose must be ready to promote their interests and ours when the balance shifts in their favour. Today, they are not."
The contradictory statements do not bode well for US interests. Coming on top of the White House's perceived frailty in Egypt where former president Hosni Mubarak has just been released from prison against Washington's wishes, Obama's failure to take a lead in Syria makes him look weak and uninterested in the affairs of the Middle East.
While that is unfair - Obama has made no secret of his belief that military action in Syria will solve nothing - it does make a nonsense of his red-line pledge. And that weakness will be exacerbated if it is proved that Syrian forces used chemical weapons last week, as it will create a perception that al-Assad ordered the attack in the certain knowledge that the US would not retaliate.
As the crisis deepens it is quickly turning into a no-win situation for Obama. Within the US there is little public appetite for intervention. Too many people have suffered as a result of the recent operations in Iraq and the continuing US presence in Afghanistan, and Obama has shown himself to be cautious in handling his country's foreign affairs. At the same time senior commanders have pointed out that other options such as arming the rebels or ordering selective air strikes will not solve the problem and might even make it worse.
Normally an incident of this kind would be passed to the United Nations Security Council but the council is already hobbled by the presence of Russia and China, which are both opposed to any intervention. After Britain's Foreign Aecretary William Hague made an urgent request for an emergency session to be opened the UN response was a call for "clarity" - hardly a statement of intent.
Even the presence of UN weapons inspectors in Damascus is unlikely to be helpful as they are only allowed to inspect three different areas and are under the jurisdiction of the Syrian government.
In a rare and unexpected move, Russia's foreign minister, Sergei Lavrov, joined forces with his US counterpart, Secretary of State John Kerry, in calling on the Syrian government to permit an inspection but this was balanced by a further statement saying that it was "up to the opposition to ensure safe access for the mission to the site of the alleged incident".
Although Lavrov opened up the possibility of co-operation he then closed the door by insinuating that the attack might not have been mounted by the government but by other forces involved in the conflict.
Faced by that kind of obduracy it is difficult to imagine any immediate action from either the US or the UN. It does not help that Washington's relations with Moscow are at an all-time low following Obama's cancellation of talks with President Vladimir Putin at next month's G20 Summit in St Petersburg.
With neither side willing to make any concessions over the diplomatic spat caused by Russia's decision to give political asylum to whistleblower Edward Snowden they are unlikely to find common ground over the Syrian crisis, which has been simmering for almost three years.
The failure to make any progress leaves the Middle East in a parlous state and matters could get much worse. Because the evidence of the use of chemical weapons is overwhelming, it is now almost immaterial which side was responsible. A taboo in modern warfare has been broken - the use of chemical weapons against civilians - and everything points to this being the worst poison-gas attack since Saddam Hussein's slaughter of Kurds in Halabja in 1988.
And just as thousands of civilians were killed or displaced in the war which followed, so is the Syrian civil war producing its own innocent casualties. According to the UN 1.7 million people have been forced to flee from Syria since the beginning of the fighting, a figure which officials describe as "a shameful milestone".