The reason the US won't sign no-first use agreement is the situation where Russia troops start an invasion of Europe (or the Chinese and North Koreans invade So

The reason the US won't sign no-first use agreement is the situation where Russia troops start an invasion of Europe (or the Chinese and North Koreans invade South Korea, etc.). So there are scenarios where the President is not the one declaring war, but responding to aggression, but would still be the one using nuclear weapons first. Either way you'd have to give the President the keys to the nuclear arsenal so that we'd have a second strike capability.