Shortly after the United States entered World War II, the Allies agreed that the European Theater would have priority in the war over the Pacific Theater. The Marine Corps' war was in the Pacific and all war plans regarding the employment of Marines reflected that. Nevertheless, because the Marine Corps was part of the naval establishment, it had responsibility also for furnishing men to parts of the Navy assigned to the European and Mediterranean theaters and to the operations conducted there. These were not large Marine formations, but were, for the most part, individual Marines and small detachments assigned to guard duty at the barracks and naval operating bases established in the United Kingdom, or men assigned as "sea-going" Marines in the detachments of the large fighting ships. Another category was filled by those intrepid Marines who volunteered for duty with the Office of Strategic Services (OSS), to undertake covert operations with the underground against the Nazis in the occupied countries of Europe and North Africa.