The War Powers Clause is found in Article I, Section 8, Clause 11 of the US Constitution. It reads, "The Congress shall have Power... To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water." Article II, Section 3 explicitly names the President as the Commander in Chief of the armed forces. However, the power to raise and support the armed forces is reserved for Congress under Article I, Section 7, Clauses 12-16.
During the debate over the content of the US Constitution, convention delegate James Madison proposed that the original terminology "make War" be changed to "declare War" with the goal of ensuring the President maintains the power to dispel acts of aggression. The delegates ratified the change. The role of the President in Article II, coupled with the intentionally ambiguous terminology of the War Powers Clause, has led to an on-going controversy over the proper process for initiating an international conflict under the US Constitution.
Congress has formally declared war five times, with each of these came following a request for such a declaration from the president. Several other military attacks administered by the president have operated under the approval of Congress, including the recent wars in Afghanistan and Iraq. These wars started without Congressional declarations, but received approval after the President petitioned for it under the requirements of the War Powers Act of 1973. The Vietnam War was initiated and conducted under three separate presidents before Congress passed this Act in an effort to force the Executive Branch to seek permission from and involve the Legislature in on-going conflicts.
There have been legal challenges to conflicts initiated without Congressional declaration. In 2003, a lawsuit was filed against President George W. Bush and Secretary of Defense Donald Rumsfeld. The lawsuit, filed by members of the military and their families, alleged that Bush and Rumsfeld did not have the power to initiate a war against Iraq. This lawsuit was dismissed by the courts for being premature, due to there being no set proof that an actual war had been initiated. The Center for Constitutional Rights, with several members of Congress, filed a similar lawsuit against President George H.W. Bush in 1990, by alleging that he did not have the power to declare war against Iraq. This lawsuit was dismissed for similar reasons.
Most recently, several members of Congress filed a lawsuit in 2011 against President Barack Obama for allegedly declaring war against Libya without Congressional approval. This lawsuit was thrown out for failure to prove that there was not a legislative remedy, in light of Congress' power to end the conflict by vote.