Bush also had the approval of Congress. But Obama has the approval of the Arab League. (Or at least, he did until the action actually started.)
None of which is to say that Obama’s action is illegitimate. But it’s very interesting how Bush was condemned for his “unilateralism” when he was actually much more multilateral than Obama.
UPDATE: A listing of the coalitions for major recent US military actions. This is the smallest of all — smaller even than Kosovo.