Making the war against Islamic State lawful
For reasons Ilya Somin explained here, U.S. military action against the Islamic State is currently illegal; it violates the Constitution and the War Powers Act, and it can't reasonably be justified as covered by Congress' 2001 authorization of the use of military force against Al-Qaeda and its enablers.
Respecting constitutional and legal niceties, especially when it would involve negotiating or compromising with Republicans in Congress, hasn't exactly been the Obama administration's strong suit. But now the administration has a golden opportunity to make any future military action against Islamic State lawful. The Islamic State has attacked France. France is a NATO ally. The president could invoke the NATO treaty, which obligates the United States to come to France's aid. The war against the Islamic State would rest on sound legal authority, with no congressional involvement (beyond funding, which Congress is already doing) needed.
I am not, however, holding my breath.