Munk applied for American citizenship in 1939 after the Anschluss and enlisted in the ski troops of the U.S. Army as a private. This was unusual as all the other young men at Scripps joined the U.S. Naval Reserve. Munk was eventually excused from military service to undertake defense-related research at Scripps. He joined several of his colleagues from Scripps at the U.S. Navy Radio and Sound Laboratory, where they developed methods related to amphibious warfare.
Their methods were used successfully to predict surf conditions for Allied landings in North Africa, the Pacific theater of war, and on D-Day during the Normandy invasion. Munk commented in 2009, “The Normandy landing is famous because weather conditions were very poor and you may not realize it was postponed by General Eisenhower for 24 hours because of the prevailing wave conditions. And then he did decide, in spite of the fact that conditions were not favorable, it would be better to go in than lose the surprise element, which would have been lost if they waited for the next tidal cycle two weeks.”