Saikrishna Prakash, Univ. of San Diego, has posted an article, forthcoming in the Cornell Law Review, Unleashing the Dogs of War: What the Constitution Means by 'Declare War'. Here's the abstract:
Though a good deal of war powers scholarship makes claims about the original meaning of "declare war," these works do not consider a host of sources pertinent to the inquiry. This Article considers these materials and sheds new light on two questions. Does Congress's power to declare war encompass the authority to decide whether the nation will wage war? Relatedly, does Congress's power to decide whether the country will wage war extend to those situations where another nation already has declared war on the United States? This Article answers both questions in the affirmative. In the eighteenth century the power to declare war was a power to decide whether a nation would wage war. Materials from that era reveal that any decision to wage war, however expressed, was a declaration of war. The commencement of warfare was regarded as the surest declaration of war. Because the Constitution grants Congress the power to declare war, only Congress can determine whether the nation will start a war or enter a war against a nation that already has declared war against the United States. Necessarily, the President cannot unilaterally choose to wage war against another nation, even when that nation has already declared war.