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We start with Article II of the United States Constitution, which outlines the powers and duties of the Executive. The Vestment Clause, found in Section I, states that "The executive power shall be vested in a President of the United States of America"3. This simple sentence has been the source of more debate and controversy than one might think. The question being, what exactly is "executive power"? Even if we go on the strict constructionist philosophy that it is the duties and powers outlined elsewhere in the document, that still leaves room for quite a bit of interpretation in the latter sections. The most important implication of this clause though is that the President of the United States may act unilaterally without anyone else's consent if he (constitutionally) deems the action to be within the realm of his executive powers. This idea that the president can act unilaterally on behalf of the entire executive branch forms a school of thought called Unitary Executive Theory4. Many constitutional scholars subscribe to the Unitary Executive Theory, but where there is disagreement is on the exact scope of what constitutes executive powers.
The applicable executive power is found in Section II. The first clause establishes the President as the Commander in Chief of the United States Armed Forces5. But what does that entail? As Alexander Hamilton wrote in Federalist #74, "The direction of war implies the direction of the common strength; and the power of directing and employing the common strength, forms a usual and essential part in the definition of the executive authority"6. This is Hamilton invoking the idea of plenary authority. What this essentially means is that Hamilton would vest in the President complete unitary power to deploy and command the military as he or she sees fit in times of "war". In the case of World War II, this would give Roosevelt the power to exercise complete military control without necessarily the immediate consent of congress if he deemed the danger to be "imminent" as stated before.