Chennault's Flying Tigers had occurred to me as an example - but what about people who fight under a foreign flag for ideological reasons rather than money ?
I'd say that most Americans who came to help us before US involvement in both World Wars had joined the Canadian Army, and several were awarded the Victoria Cross in the Great War*. On one wise, the American Eagle Squadrons in the RAF were just another national grouping, alongside Czech, Polish, Free French, Canadian, Aussie, Kiwi South African, Rhodesian squadrons. However, the Americans had no dog in the race before December 1941, and most transferred to the Mighty Eighth once it got into action.
Red Morgan is a wonderful example - he tried to join the USAAF in 1938 but was rejected due to his poor educational record. He later joined the RCAF and after qualifying as a pilot he came to England and flew as a Sergeant pilot. On transferring to the USAAF, he was not given a commission (The Americans seemed to think that a man had to be commissioned in order to fill certain crew positions) and so he was initially graded as "Flight Officer" which was a curious position, similar, I suppose to a British Warrant Officer. In 1943 on his fifth US mission he was co-pilot in a B-17 of the 92nd Heavy Bomb Group and won a Medal of Honor in what I always consider to be the most remarkable flying MoH of any war.**
https://en.wikipedia.org/wiki/John_C._Morgan.
If you look at the photo in the link you'll see that Morgan eventually got his commission. You'll also note that he is wearing RAF pilots wings on the right breast which was the norm amongst USAAF airmen who had previously served in the RAF.
Then we come to organisations like the 15th International Brigade (including the Abraham Lincoln Battalion) during the Spanish Civil War - were they not ideological mercenaries ? (and there were ideologues who fought for the Nationalists too, though nowhere near as many.)
The list goes on. I am really confused about how to define a mercenary.
*. Four out of the five US citizens awarded the Victoria Cross were serving in the Canadian Army in the Great War. But the fifth - now here is an interesting thing - was Ordinary Seaman William Seeley, Royal Navy, HMS
Euralyus, who won his Cross in Japan . . . . . in 1864 !!! (Bombardment of Shimonoseki). What was this New Englander doing in the Royal Navy in the middle of the Civil War ?
**Whilst Red Morgans MoH pips all other flying US and all but one flying VC , (in my estimation) I do believe that the Cross awarded to Flight Sergeant Norman Jackson takes the cake
https://en.wikipedia.org/wiki/Norman_Cyril_Jackson
He was Flight Engineer in a Lancaster attacking the famous ball bearing factories at Schweinfurt on the night of 26/27 April 1944. He had been wounded by splinters after a night fighter attack which set an engine on fire. Jackson attached a parachute, grabbed a fire extinguisher, and climbed out of a hatch in the top of the fuselage -
this at 20,000 feet , at 140 mph in the middle of the night ! He managed to cling on, climbed down on to the wing, and started to try and insert the extinguisher into the engine cowling, suffering burns to his face and hands. At this point, with him out on the wing, the night fighter attacked again. His parachute had deployed accidentally as he had climbed out, and other crew members were clinging to the canopy inside the aircraft, and when they pushed it out it caught fire and dragged Jackson through the flames. The last the crew saw of him, was his smouldering parachute in the night sky.
Amazingly he survived, and fortunately the rest of the crew managed to parachute to safety, so this unbelievable episode was reported after PoWs were repatriated.
It all goes to make you wonder how many Red Morgan and Norman Jackson stories there were which could never be told.
Thanks for reading - I am rambling a bit this evening as thoughts of VE Day 75 are filling my mind.