We've had extortion and murder as well, your argument doesn't really help.
Modern nation states can 100% regulate themselves on the use of irregular forces - there are situations for them, and many not.
They could be doing some enhanced security, some types of armed guards ... but not anything tactical, certainly not anything mission oriented or directed kinds of violence.
Mercenaries may always exist, but American ones don't have to. Participation in them should be made illegal, and all these private military companies should forbidden to operate until they find another line of business or liquidate themselves. If some former soldier or former general wants to become a mercenary, let him renounce his citizenship and go find a new home.
The only military forces in the US should be explicit parts of official US military.
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 §§ 956, 958, and 960.
