The increasingly unilateral nature of U.S. foreign policy — especially in its lethal aspects — has been something of a cooperative project in constitutional dysfunction. For all that presidents of both parties are loath to ask the legislative branch to exercise its power to declare war under Article 1, Section 8, of the Constitution, many lawmakers are equally resistant to being committed by an actual up or down vote in a way that might force them to take responsibility for the next bloody and unpopular fiasco. House Speaker John Boehner asked President Obama to “make the case to the American people and Congress for how potential military action will secure American national security interest..." and whether action against Syria might require Congress to authorize more money, but he hasn't called for an actual vote. Pointedly, he hasn't signed on to Rep. Scott Rigell's (R-VA) letter (PDF) demanding that President Obama "consult and receive authorization from Congress before ordering the use of U.S. military force in Syria." That letter is now up to 140 signatures from lawmakers of both major parties, but that's still a minority of the membership of the House of Representatives.
President Obama, for his part, was once an enthusiastic believer in the idea that "[t]he President does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation." But that was before he took up residence in the White House and faced the possibility that Congress, like the U.K. parliament, might say "no."