Eagle Squadron

"''From near and far they arrived, joined the force

Ready to serve the allied command

Sent into training though they already earned their wings

They were ready fly, they were fit for the fight''"

- Sabaton, "Aces in Exile"

Country A is at war with Country B. Even though Country C isn't involved in the war, many citizens of Country C support Country A. Some even go so far as to join in the fight on the Country A side, either as individuals enlisting in the Country A Armed Forces, or as free agents in units formed entirely of citizens of Country C that are affiliated with the Country A forces.

These people aren't Hired Guns, although the money may be a motivation. They genuinely believe in the cause they're fighting for and may well bring their own military experience to the battle.

Contrast Fighting For A Homeland. Compare the Legion of Lost Souls, which is a more specific sister trope based on a real life unit.

Named after the American volunteer squadrons which served in the Royal Air Force during the first two of the seven years of World War Two. They were recalled when the US was forced into the war proper.

Anime and Manga

 * Happens fairly often during The Five Star Stories, most notably during the Colus/Hagooda conflict of J.C. 2989 in Books II & III, which saw both sides recieving large amounts of support from foreign soldiers. While Hagooda initiall has the upper hand due to being secretly backed up by the governments of two major powers who have a beef with Colus, after the king is mortally wounded and his Fatima partner is killed by one of the "Mercenary" batallions serving Hagooda, Colus sees a massive influx of volunteers from allied nations coming in to join the fight. What makes this interesting is that some of said volunteers are actually members of foreign royalty or high officials, namely Emperor Amaterasu of Delta Belune and President Mission Routh of the Trun Union, who are participating due to a debt of honor they owe the king for his assistance during the events of Book I, but decline to directly involve their nations' military forces in the war, bringing only select members of their Imperial or Presidential Guards.

Comics

 * The Steve Canyon comics (both comic books and newspaper strips) by Milton Caniff used this. The newspaper comics placed Steve in charge of "The Dragonflies" flying against a "puppet government" in China for an extended storyline starting here.
 * Several strips from the British Commando comic book featured Czech or Polish pilots who flew for the British air force in the Second World War.
 * DC's Blackhawk Squadron is a Multinational Team version of this, to an extent. Many of the Blackhawks were, however, citizens of countries at war with the Axis powers, or occupied by them.
 * Chuck fits the trope most exactly, as an American who joined the RAF before becoming a Blackhawk.

Live Action TV

 * Babylon 5 has a number of variations (but few straight examples).
 * The Anla'Shok, also known as the Rangers, are a Minbari covert operations organization operated by the Religious Caste, who boost their numbers leading up to the Shadow War by recruiting large numbers of human volunteers, unbeknowest to the Earth government. In the show's final season, the Rangers begin to recruit from the other races as they begin to take on the role of interstellar peace keepers.
 * When the command staff is forced to purge large numbers of their security staff after the Nightwatch attempted to seize control, they are able to make up for it with large numbers of Narn volunteers.
 * When Captain Jack Harkness first meets the Doctor in Doctor Who, he is in one of the trope-naming historical Eagle Squadrons, having taken a dead guy's identity according to the Torchwood episode "Captain Jack Harkness".

Film

 * Dark Blue World is a Czech-made film about pilots who enlisted in the British Royal Air Force after Czechoslovakia was conquered by Nazi Germany.
 * For some reason Pearl Harbor has a scene with Ben Affleck flying a Spitfire for the RAF in the Battle of Britain, probably to explain how the British were able to hold off the Luftwaffe. He's ostensibly a member of the actual Eagle Squadron, but the codes on the side of his airplane are (confusingly) those of 303 Squadron -- which was one of the famous all-Polish units! Was Affleck actually meant to be Polish?
 * Well, judging by the fact that they had him transfer there from the US Army Air Force while the US was still neutral, I don't think they were even trying. For contrast, during the Battle of Britain, Americans who wished to fight for Britain had to give up their citizenship and swear an oath of loyalty to the king. It wasn't until after the battle was over that American citizens could fight for Britain while retaining their citizenship, but even in 1941, a year after the Bo B was over, you still couldn't be an active American serviceman fighting for the British. Yes, I do hate this movie.
 * Flyboys about the Lafyette Escadrille in World War One.
 * The Great Escape has Hendley, who's a still member of the Trope Namer since he was shot down before Pearl Harbor and so hasn't been transferred to the USAF. Amusingly, as he's The Scrounger, his RAF uniform is in better condition than the actual British characters'.

Real Life

 * The Trope Namer is the American volunteer squadrons in the Royal Air Force during the early (i.e. American neutrality) period of World War Two.
 * In fact, the first USAAF aircraft to fight on the Western Front after America's entry into the war were British Spitfires from the Eagle Squadrons which were repainted to have American insignia on them. They transitioned to American-made airframes as the American war effort got going.
 * Another good example are the American Volunteer Group (AKA the "Flying Tigers") who flew P-40 Warhawks for China against the Japanese. Contrary to legend the AVG were mercenaries (and well paid ones at that) and didn't actually fly their first mission until four days after Pearl Harbor. They were eventually incorporated into the USAAF.
 * The Soviet Operation Zet was an earlier example in the same war.
 * In the Spanish Civil War, both sides saw many foreign volunteers, including the famous author George Orwell on the Republican side.
 * Here is the Wikipedia category on these. Some more notable ones from the list:
 * The WWI French equiv. of the Eagle Squadron, the Lafayette Escadrille.
 * Named after Lafayette of course who came and fought for the Americans while France wasn't sure whether to ally.
 * The Mahal during the Israeli War for Independence, a combination of Jews fighting for a homeland and WWII Veterans looking for adventure. The very first Israeli Brigadier General had been a Colonel in the US Army, much of the early Israeli Air Force was made up of American and Canadians who had flown against the Luftwaffe.
 * The French Foreign Legion, depending on what the French are doing at any given time.
 * The 13th international Brigade of the Spanish civil war.
 * Swedish volunteers fought against the Soviets in Finland during the Winter War.
 * The Yugoslav Wars saw foreign volunteers fighting on all sides. Arabs fought on the Bosnian Muslim side, while Greeks and Russians fought for the Serbs. There were cases of neo-Nazi volunteers fighting for the Serbs or Croats.
 * A number of foreign volunteers served in the Rhodesian security forces during the 1970s.
 * After Nazi Germany conquered their countries, many Polish and Czechoslovakian pilots traveled to the UK and decided to fly for the British Royal Air Force. The top-scoring RAF pilot of the Battle of Britain was Czech, and the pilots of the top-scoring RAF squadron were Polish.
 * Just about any war fought by the USA but not Canada, or vice versa, lead to men from the non-participating country crossing the border to enlist. Most notably with the early years of WWI, WWII and Vietnam.
 * The Texas Revolution featured American militia regiments joining the young Army of the Republic of Texas. Santa Anna (dictator of Mexico at the time) regarded them as pirates and mercenaries as they had no claim for fighting, and this lead to the most notable massacres of the time aginst both the American volunteers and the white Texan settlers.
 * The Czechoslovakian Legion (on White side) and the International Units of the Red Army (on Red side) during the Russian Civil War. Many of them were WWI ex-POWs, but all POWs were free to leave Russia after 1917. These guys stayed, so they qualify for this trope.
 * The Czechoslovakian Legion was more of a subversion. It was like this in the beginning, but afterwards they decided to leave this place and go home. To do this, they betrayed Admiral Kolchak.
 * In the Polish-Soviet War, the Kościuszko Squadron was a squadron of American volunteers fighting for the Polish.
 * Escuadrón 201, also known as the "Aztec Eagles", of the Fuerza Aérea Expedicionaria Mexicana. Mexico declared war on Germany after U-Boats began sinking Mexican oil tankers in the Gulf of Mexico. Lacking the resources to raise an expeditionary army, Mexico instead formed a squadron of volunteers who were sent to Texas to be trained and equipped as a unit attached to the US Army Air Forces. They served with distinction in the Philippines until the end of the war.
 * The Spanish Civil War saw quite a number of foreign volunteers fighting on both sides; the most famous are probably the German Condor Legion and the Italian Aviazione Legionaria (for the Nationalists) and the many International Brigades (for the Republicans).
 * The Nazis had quite a few during World War II. The last troops defending Hitler's Chancellery in Berlin were French Waffen SS of the "Charlemagne" division.
 * The Free French Normandie-Niemen fighter squadron, flying for the Soviet Union in World War II.
 * Russian pilots flew for China and North Korea during the Korean War. Because proof of Russian involvement could have escalated the conflict into World War III, the Russians pretended to be Korean and the UN forces went along with the pretense.
 * Chinese People's Volunteer Army during the Korean War.

Video Games

 * Blazing Angels: The player and their squadron start the game off as American RAF pilots flying missions in France, England, and Africa before transferring to the USAAF for the remainder of the game once America enters the war.