Given that the US is not, in fact, at war with Yemen, it doesn't seem to be legal outside the US for US citizens or people within US jurisdiction when doing any planning, etc., related to it; see, U.S. Code tit. 18, ch. 45, particularly ยงยง 956, 958, and 960.
https://www.law.cornell.edu/uscode/text/18/part-I/chapter-45