The United Arab Emirates hired U.S. mercenary firm Spear Operations Group, comprised mostly of ex-U.S. special forces, in 2015 to carry out an assassination. This kind of contracting appears to be on the rise, highlighting the privatization of many kinds of conflict, the importance of special forces, and the change in objectives from traditional military objectives to more organizational and individual objectives. U.S. law is unclear on the general subject, with clearly statements on the illegality of conspiring kill or kidnap individuals overseas, but mercenary work is not banned, nor is participation in foreign militaries. According to sources, the AUE may have given the mercenaries a special military rank to provide legal cover for what could otherwise be considered murder by a foreign national. Another question that the use of mercenaries raises is accountability. Is the US accountable for their ex-special forces? 

Source: A Middle East Monarchy Hired American Ex-Soldiers To Kill Its Political Enemies. This Could Be The Future Of War.