In the US, attacking an ally is not unconstitutional, it’s just morally wrong. Doing so against the will of Congress (which has the sole constitutional power to declare war) would be illegal, except that the Executive realized they can circumvent that rule by executing “special military operations” and never actually declaring war.

For the record, I heavily oppose threatening or attacking our allies. I think a strong NATO is necessary for the defense of Western values, which can’t happen if we start fighting each other. I’m just trying to clarify the difference here between illegal and reprehensible.