Русско-англо-русский юридический словарь

A commander-in-chief is the person or body that exercises supreme operational command and control of a nation's military forces or significant elements of those forces. In the latter case, the force element is those forces within a particular region, or associated by function. As a practical term, it refers to military competencies that reside in a nation-state's executive leadership—either a head of state, a head of government, a minister of defence, a national cabinet, or some other collegial body. (Wikipedia)

The US Constitution deliberately split responsibility, making the President the Commander-in-Chief, while giving Congress the power to declare war. (bbc.co.uk)