Tronstad, holding a military rank, had a standing order to report to the Norwegian military headquarters in Oslo in the face of a military peril. When Germany invaded Norway on 9 April 1940, he first informed his students of the situation, and then drove towards Oslo. He brought his wife and children with him, but having no intention of taking them to a possible theatre of war, he left them in the Drivdalen valley. He continued alone but shortly after, upon learning that Oslo was the first city to fall to the invaders, he stopped at Dovre. The family had a cabin in the vicinity. Here, he helped organize volunteers from local rifle associations to form a line of defence meant to repel any advancing German forces. German Fallschirmjäger paratroops did land at nearby Dombås, but were surprised by a contingent of regular Norwegian forces already in the area, who ensured a tactical victory for Norway in the ensuing Battle of Dombås.