It must be galling for him to feel forced to go to war against what he once called "a JV team." It appears no particular change is anticipated in actual attack intensity, now ongoing.
By this action he formally takes "ownership" of our latest war. It's a war against an enemy the nature of which he continues to muddle and obfuscate.
Like Neville Chamberlain in the late 1930s willfully misunderstanding the Nazi menace, Obama refuses to name radical Islam as our foe. The times demand a Churchill figure, maybe Bobby Jindal?