Having secured the Marshall and Caroline island chains, the U.S. Navy prepared to climb the next rung of the ladder leading to the Japanese home islands. The Mariana Island group was dominated by three islands in particular: Saipan, Tinian, and Guam. Saipan had been mandated to Japan under the Versailles Treaty in 1919 as reward for its nominal participation in World War I. The Japanese had intended eventually to annex the island and had initiated a rather substantial colonization effort on it. Thus for the first time Japanese civilians would be eyewitnesses to the American conquest of an island.