While running for President in 2007, then-Senator Barack Obama sponsored a resolution to require President George W. Bush to get explicit authorization from Congress before waging war against Iran. Today, the Obama White House says the war on ISIS is legally permissible without explicit authorization from Congress, because it's covered by the 2001 Authorization of Use of Military Force (AUMF) — which Obama wanted to repeal as recently as last year. In May 2013, Obama said of the AUMF:
The AUMF is now nearly 12 years old. Unless we discipline our thinking, our definitions, our actions, we may be drawn into more wars we don't need to fight, or continue to grant presidents unbound powers more suited for traditional armed conflicts between nation states. [Washington Times]
Now, the Administration still maintains that Obama still wants to repeal the AUMF, saying "The president has made clear that he wishes to take America off a permanent war footing," as a new long-term war in Iraq begins.