[House Atreides]

If the President can do it anytime, but Congress only as part of a declaration of war, why would he have that extra power?
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Because HE is the Commander-in-Chief and Congress recognized that when they wrote the law?

[Svartalfiar]

Because HE is the Commander-in-Chief and Congress recognized that when they wrote the law?

True, but that doesn't mean he can just whatever he wants. Every power in the law needs reasoning - none of it is just arbitrary. Why would the President be able to just randomly move the CG into the Navy? The preceding clause gives the answer - when war is declared.