During the Spanish Civil War (1936-'39) about 3,000 young, American men defied a prohibition by the U.S. State Department to fight against the facist Franco regime. Largely motivated by some belief in radical politics and a dedication to stop Fascism, they secretly crossed the Pyrenees to aid the rightfully elected Republican Government in Madrid. They fought alongside volunteers from 52 different nations.
The Lincoln Brigade, as they came to be known, was the first U.S. millitary unit to be led by a black commander. The soldiers were largely members of the Communist Party, although a good percentage were members of the IWW. The organization of the brigade little resembled the typical chain of command. Each division voted its superior officers, and a great effort was made to keep units informed on the political motivations for their maneuvers.
While the Spanish Fascists were gaining definite support from Hitler in Germany, and Mussolini in Italy, the soon-to-be Allied Nations, under the policy of Appeasement, seemed to be doing their best to keep supplies out of the hands of the Republicans. Only the Soviet Union and Mexico provided aid to the Republic, much of which was seized in by the French. Fighting against the army that launched the bombing of Guernica the Lincoln Brigade sustained heavy casualties.
Many of the veterans of the Lincoln Brigade went on to fight in World War II. Some were recruited into the OSS (the predecessor of the CIA) because of their experiences in Spain.
After the War, however, the McCarthyist society of America in the '50's led to the blacklisting of Lincoln Brigaders, with the absurd charge that they were "prematurely anti-Fascist".
See http://www.english.upenn.edu/~afilreis/88/abe-brigade.html and http://www.alba-valb.org/ for a fuller history. Personally, let me recomend the book The Anti-Warior, by Milt Felsen, as a account of one of the volunteers. I have met Mr. Felsen, an amazing gentleman, who after having served as a machine gunner in the Lincoln Brigade went on to produce Saturday Night Fever (you never would have guessed, would you?)