Japanese Airways began some commercial flights in 1935 using the long-range Kawanishi H6K2-L seaplane. Regular commercial flights were begun in 1940 and a regular service commenced in 1941. Commercial services ceased shortly after the start of Pacific War, but the widespread network of seaplane bases continued to be used during wartime. Pacific War The terms of the mandate required Japan not to fortify the islands. However, these terms were ambiguous and poorly-defined, specifying only that Japan should not build "fortifications" or construct "military or naval bases". From 1921 the Japanese military began making surveys and plans so that rapid military deployment to the islands would be possible in case of war. During the 1930s, the IJN began construction of airfields, fortifications, ports, and other military projects in the islands controlled under the mandate, viewing the islands as "unsinkable aircraft carriers" with a critical role to play in the defense of the Japanese home islands against potential US invasion. These became important staging grounds for Japanese air and naval offensives in the Pacific War. Kwajalein Atoll in the Marshall Islands was a major base supporting the attack on Pearl Harbor and the Battle of Wake Island. Palau was used to support the invasion of the Philippines by Japan. Saipan in the Mariana Islands supported the Battle of Guam. Truk in the Caroline Islands became the base for amphibious landings on Tarawa and Makin in the Gilberts during Japanese occupation of the Gilbert Islands, as well as Rabaul in the Australian mandate Territory of New Guinea. Kosrae was used to relocate Ocean Island workers evacuated Majuro in the Marshall Islands was used in air strikes against Howland Island. Jaluit Atoll, also in the Marshall Islands, was the base from which the IJN seized Nauru and Ocean Island (now known as Banaba Island). The Imperial Japanese Army also utilized the islands to support air and land detachments. In order to capture the islands from Japan, the United States military employed a "leapfrogging" strategy which involved conducting amphibious assaults on selected Japanese island fortresses, subjecting some to air attack only and entirely skipping over others. This strategy caused the Japanese Empire to lose control of its Pacific possessions between 1943 and 1945. The League of Nations mandate was formally revoked by the United Nations on 18 July 1947 pursuant to Security Council Resolution 21, making the United States responsible for administration of the islands