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World War I and later

Soon after graduating from college, Capra was commissioned in the United States Army as a second lieutenant, having completed campus ROTC. In the Army, he taught mathematics to artillerymen at Fort Point, San Francisco. His father died during the war in an accident (1916). In the Army, Capra contracted Spanish flu and was medically discharged to return home to live with his mother. He became a naturalized U.S. citizen in 1920, taking the name Frank Russell Capra. Living at home with his siblings and mother, Capra was the only family member with a college education, yet he was the only one who remained chronically unemployed. After a year without work, seeing how his siblings had steady jobs, he felt he was a failure, which led to bouts of depression.

Chronic abdominal pains were later discovered to have been an undiagnosed burst appendix. After recovering at home, Capra moved out and spent the next few years living in flophouses in San Francisco and hopping freight trains, wandering the Western United States. To support himself, he took odd jobs on farms, as a movie extra, playing poker, and selling local oil well stocks.

During this time the 24-year-old Capra directed a 32-minute documentary film titled La Visita Dell'Incrociatore Italiano Libya a San Francisco. Not only did it document the visit of the Italian naval vessel Libya to San Francisco, but also the reception given to the crew of the ship by San Francisco's L'Italia Virtus Club, now known as the San Francisco Italian Athletic Club.

At 25, Capra took a job selling books written and published by American philosopher Elbert Hubbard. Capra recalled that he "hated being a peasant, being a scrounging new kid trapped in the Sicilian ghetto of Los Angeles.... All I had was cockiness—and let me tell you that gets you a long way."