From Americans at War in Foreign Forces: By 1937, the proxy nature of the Spanish Civil War had taken on the specter of a world struggle between harsh authoritarianism that some referred to as fascism and a freedom to work together for economic and personal fulfillment that some referred to as Communism. In the United States, as elsewhere, the struggle was fueled by the economic turmoil of the Great Depression. An attraction to the civil war in Spain seemed a natural result for some . . .
For whatever mix of reasons, 2800 Americans entered the 15th International Brigade on the side of the Loyalists, part of an international group estimated at 40,000 to 60,000. Thirteen hundred Canadians also volunteered, defying a Foreign Enlistment Act of 1937, and a mix of Americans and Canadians eventually filled the American George Washington and Abraham Lincoln battalions and the Canadian Mackenzie-Papineau Battalion. As in the prelude to American involvement in World War I, they were fighters and nurses, ambulance drivers and engineers. And, as before, many enlisted in the cause under ambiguous identities. They may have been seen as the leading edge out of the American neutrality that was increasingly debated as conflict in Europe escalated.
The Yankee Squadron From Americans at War in Foreign Forces: It appeared, however, that when Bert Acosta and his associates arrived in Valencia to fly [as the Yankee Squadron] on the side of the loyalists, or Republicans, against the Nationalist forces of Francisco Franco, it was not primarily under any great political motivation. They had met tough qualifications for the work and would be paid $1500.00 per month, plus $1000.00 per plane shot down and be given a $2000.00 insurance policy. They worked effectively on the Republican side, and Acosta was believed to be an unnamed Ace who led very successful air battles against Nationalist planes over crucial supply lines early in December 1936.
[The U.S. State Department demanded that they return.] Two of those who did return from Spain, and rather quickly, were the American flyers Bert Acosta and Edward Schneider. They were met by federal subpoenas upon their arrival in New York on the ocean liner Paris . . . The stories the men told offered a glimpse into the sketchy beginnings of the Spanish Civil War. The bombers they flew were domestic sport planes with holes cut in their floors for the dropping of bombs; and the planes of their adversaries weren’t much better. Sometimes the only guns available were those the pilots carried in their own pockets, though the Spanish pilots in particular seemed to be heedless of any danger. Food was not reliably available, and one could never be sure of the true loyalties of those one spoke with, so it was best to remain quiet.