https://twitter.com/lizthegrey/status/1497843649668739072 https://twitter.com/lizthegrey/status/1497843649668739072 is a short thread on basically this subject,

https://twitter.com/lizthegrey/status/1497843649668739072 https://twitter.com/lizthegrey/status/1497843649668739072 is a short thread on basically this subject, but no clear answer. 18 USC 958 makes illegal "within the jurisdiction" of the US to accept "a commission" to fight against any state "with whom the United States is at peace." (As the thread points out, public perception and the willingness of prosecutors to go after you are probably more relevant from a mechanics/plausibility standpoint.)