During the war, he volunteered for dangerous guerrilla operations which frequently required him to work behind enemy lines for up to weeks at a time.

Working with International brigaders, Alex Kunslich and Irving Goff, Aalto was trained by Soviet instructors in the use of pressure-sensitive explosives to destroy railroad tracks, bridges and power lines.

[6]: 167 At the end of 1937, Aalto took part in the Battle of Teruel, working behind enemy lines again with Kunslich, Goff and Spanish guerrillas.

[1] In September 1938, with a Republican defeat in sight, the Abraham Lincoln Battalion was withdrawn from the front line and shortly afterwards disbanded.

During his time in Spain, Aalto wrote: "A soldier who is politically conscious that he is right and who has a feeling of community with his society... will do his job well.

Goff and other OSS Lincoln veterans reported the fact to the organization's head, General William Donovan, requesting for him to be removed from their team.