With few modifications, this method of moving classified mail abroad continued until 1918 when the War Department established the Military Postal Express Service, consisting of 70 officer and enlisted Army personnel, divided into an Overseas Service and a European Service. Despite some turbulence, this continued until the early days of World War II when the War Department activated the Army Courier Service to move classified material between the War Department and various theaters of operation. Meanwhile, the Navy created the Officer Messenger Service and the Army Air Corps operated an Air Courier Service to move cryptographic materials. Frequently, couriers from all three services flew together on the routes.