I had thought that the White House would try to spin Barack Obama’s pledge to seek Congressional authorization as his plan all along.  After all, the President had never publicly declared he didn’t need it, although his aides were certainly telling the press that.  However, the AP reports that this was a change of mind within the last 24 hours:

Senior administration officials say President Barack Obama planned to take military action against Syria without congressional authorization, but told aides Friday night that he changed his mind.

Obama announced to the public Saturday that he wanted to launch a military strike, but that he first would seek approval from lawmakers.

The administration officials described a president overriding all his top national security advisers, who believed Obama had the authority to act on his own.

I suspect the collapse of British support was the final straw for Obama.  That left him much too exposed, and much too isolated abroad to go it alone at home.