At age 17, he left his parents' home in Paris, France, to join those fighting against Francisco Franco's nationalist forces in the Spanish Civil War.

[1] He initially went to the offices of the French Communist Party, but was turned away, and then denied by the Socialists as well.

He was sent to Barcelona, then received one week's worth of military training before being sent to the front.

[2] During the Civil War, Sossenko changed his name to Georges Jorat to avoid being found by his parents, and fought as part of the Sébastien Faure Century,[3] the French-speaking contingent of the Durruti Column.

[2] In 2004, Sossenko published a Spanish-language book titled Aventurero Idealista.