The separation of powers doctrine incorporated into the US Constitution includes the ability to declare and enter war. Article I, Section 8 of the Constitution gives Congress the ability to declare war. Article II, Section 2 grants power of the US armed forces to the President as Commander-in-Chief.
Providing a single person with the ability to go to war was an issue of grave concern for the participants of the Constitutional Convention of 1787. Analysis of both the Federalist Papers and the debate record at the Convention verify [PDF] that the framers' chose the words of the Constitution with specificity, expecting each branch of government to retain unique powers regarding military action. Early conflicts tested [PDF] the willingness of a president to expand war power capabilities. President Washington understood that the powers authorizing an offensive operation lay in the hands of Congress and only personally approved defensive maneuvers against Native Americans. Washington also approved defensive military action against agitators during the Whiskey Rebellion, as civil unrest moved beyond mere unhappiness to shootouts and property destruction between 1791 and 1794.
During the Napoleonic Wars, the US passed trade embargoes to circumvent hostile actions by Great Britain and France. Later, the US requested that each country honor neutral trade, but Great Britain continued to thwart commerce and impress US sailors. On June 1, 1812, President Madison implored Congress to issue its first formal declaration of war. The House of Representatives passed the declaration on June 4 and the Senate passed it on June 17, 1812. Madison signed it on June 18, 1812.
The Kennedy, Johnson and Nixon administrations pushed the boundaries of executive power under war powers through the Vietnam War, and Congress responded by passing the War Powers Resolution in 1973. It required the President to tell Congress within 24 hours whenever troops were pledged, and forced the President to remove all troops should Congress not grant an extension within 60 days.
According to a report [PDF] prepared by the Congressional Research Service in January 2007, the Bush Administration drafted a joint resolution post-September 11, 2001 that would have "...authorized the President, without durational limitation, and at his sole discretion, to take military action against any nation, terrorist group or individuals in the world without having to seek further authority from Congress." Congress declined the resolution as presented, but worked quickly to draft a more amenable version. The Authorization for Use of Military Force passed the US House of Representatives and the US Senate on September 14, 2001 and signed [PDF] into law by US President Bush on September 18, 2001. This resolution limited force to actors responsible for the 9/11 attacks and required the President to act within the powers authorized by the War Powers Resolution.