Leading powers would not be touchable by private armed forces, but there's a score of little ratbag tyrannies that could use a quick de-lousing.
The U.S.'s Logan Act forbids any U.S. citizens from participating in such an adventure.
The original 'filibusters' were a private army that went to Nicaragua in the late 1800s to do just that - quite a discouraging story, really. ;^)
Well now someone missed their opportunity while Mugabe hotfooted it to Tuh-Ray-Sah's country way back when. Whoops. Talk about MISSED chances ;-) BTW, when Mugabe visits SA the security team casts a wide berth complete with FOOD TASTERS. BWAHAHAHA. C'mon now sheesh.