52.000 Spaniards, Republican refugees in France in 1939, were involved in the Foreign Workers Units as workforce to build fortifications in the famous and impressive (and then unuseful) Maginot Line. This line was a military border thought to stop the invasion of Germans after the outbreak of WWII.
The voluntary workers of the Foreign Workers Units found this as an opportunity to escape the refugees camps in the South of France. They had a better treatment, better food, a small salary and the opportunity to fight against Nazism. Most of those Spaniards never came back. Germans crossed the famous Maginot Line and the Spaniards ended up in extermination nazi camps.