Because National Guard units had been mobilized for over a year in December 1941, they were among the first to enter combat in the following months. California's 251st Coast Artillery Regiment and Hawaii's 298th Infantry Regiment took part in the defense of Oahu on 7 December 1941, during the Japanese attack on Pearl Harbor. New Mexico's 200th Coast Artillery Regiment and two tank battalions made up of National Guard units from several states were part of the defense of the Philippines, with more than half of these men dying as prisoners of war of the Japanese.