Also the old term of “sheep dipping” officially resigned from the military and now a private citizen. Unofficially still on US government payroll but just under CIA or another agencies control. That agency will look after that soldiers pay benefits promotions. CIA even set up its own insurance company when it recruited pilots for its covert air force from US military. Think there has been around 50 US private citizen deaths in Ukraine since war started. Around 40 were ex military.

Example was one way CIA hired pilots from US military for its Air America covert air force. Officially left the military. Unofficially now a pilot for a civilian company that was owned by CIA. Company would bring in legal business with normal government contracts( mail runs etc) whilst also covert supply runs into China for example

Book Air America is worth checking out. In fact similar practice was used to help China before US officially entered WW2.

Air America: The History of the CIA’s Covert Airline

Ukraine has a Foreign volunteers Legion. It’s officially part of Ukraines Armed Forces. So can just fill those ranks.

From this website

https://ildu.com.ua/

Will I be considered a mercenary or a criminal if I join the Legion? No, you will become a fully legal serviceman of the Armed Forces of Ukraine, just like all other Ukrainians and non-citizens who bravely serve. However, we highly recommend checking with your local laws about your state’s stance towards people who served in other countries’ military.

If you haven’t checked it out. Then Projects Minaret and Shamrock. Then Senator Church committee in 1975. Then notice history repeating itself with Thin thread, Trailblazer and Stellar Wind