[House Atreides]

I always understood it to be only so during time of war. I do not think it would apply here and not even after a declaration of emergency.
****************************************************
It’s clear that Congress, on its own, can only affect such a transfer if they do so as part of a declaration of war.

I’m not an attorney or judge but it appears (to me, anyhow) that the Commander in Chief is not similarly restricted by the law. It depends upon how you read 14 U.S. Code § 3 (b).

[Svartalfiar]

It depends upon how you read 14 U.S. Code § 3 (b).

Occam's Razor. I don't have time to look up any contemporary discussion on the law, but the simplest reading is that both can only do so in a time of war. (And by inference, only in support of said war.) If the President can do it anytime, but Congress only as part of a declaration of war, why would he have that extra power? What is accomplished by allowing the President to do this at any time? Why would Congress NOT be allowed to?

OR, you read the sentence as a declaration of war allows -Congress or the President- to Navytize the Coasties, and there's no confusion or ambiguity. Without other information, that's the reading that makes the most sense.