The power of Congress:
“The Congress shall have Power...To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water...To make Rules for the Government and Regulation of the land and naval Forces.” (U.S. Constitution, Article I, Section 8)

The Power of the Executive:
“The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States” (U.S. Constitution, Article II, Section 2)
(The Constituion which describes the near plenary powers of Congress and the minute powers of the Executive concerning war and military conduct.)

Related Categories
Federal Government: Executive