commander-in-chief

According to Article II, Section 2, Clause I of the Constitution, the president of the United States is “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.”
president of the United States

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Discourse on the Method, part 5, ¶ 9-4

see also

Wikipedia

Commander-in-chief (Wikipedia) A commander-in-chief or supreme commander is the person who exercises supreme command and control over an armed force or a military branch. As a technical term, it refers to military competencies that reside in a country’s executive leadership, a head of state, head of government, or other designated government official.