Any military response to alleged chemical weapons use by the Syrian regime of Bashar al-Assad will likely rest squarely on US shoulders after Britain's refusal to commit its forces.
Once again at the spearhead of a possible lightning strike against Syrian military targets, the United States has reinforced its naval units in the eastern Mediterranean and could get help from France.
Five US destroyers carrying hundreds of cruise missiles between them are believed to be readying launches against Syrian munitions depots and command and control hubs.
The USS Stout has been deployed to join sister ships Mahan, Ramage, Barry and Gravely, a US defence official said on Thursday.
The navy does not say how many missiles are carried by each ship but they are believed to number around 40 per vessel according to military analysts.
The flagship of the US Sixth Fleet, the USS Mount Whitney, is based in Gaeta, Italy.
US forces are also stationed at air bases in Izmir and Incirlik, Turkey, and long-range bombers could eventually be sent from bases in North America.
Ships attached to the 26th Marine Expeditionary Unit are now in port in the United Arab Emirates, and two aircraft carrier battle groups built around the USS Truman and USS Nimitz are cruising in the northern Indian Ocean.
The only European nation prepared to intervene at this point is France, which has ships and aircraft standing by.
French aircraft can be equipped with Scalp missiles that could reach targets in Syria.
In Djibouti, France has positioned seven Mirage 2000 combat jets and six Rafales are stationed in Abu Dhabi.
Naval assets include frigates, submarines and the aircraft carrier Charles de Gaulle, which is based in Toulon, southern France.
Meanwhile, one of Syria's neighbours is Turkey, which has the second biggest NATO ground force after the United States and could play a major role if an intervention extended beyond limited air strikes.
The Turkish army has a total of 510,000 troops and the air force has 354 combat aircraft, essentially US-built F-16s.
Patriot anti-missile batteries have also been positioned in southern Turkey and are manned by troops from the United States, Germany and the Netherlands under a NATO mandate to protect the country from any spillover of the fighting in Syria.
Gulf countries are not expected to provide military forces or equipment but would likely allow US and French forces to operate from their bases in the region and provide diplomatic cover for an intervention.