Spain and Mexico Monterey served as California's administrative center and chief port. It naturally became the initial target of Americans who hoped to extend the United States to the Pacific Coast. The port was seized on 19 October 1842 by Commo. Thomas ap Catesby Jones in the mistaken belief that war had broken out between the United States and Mexico. He returned it the following day upon discovering that his information was erroneous. The town fell to American forces a second time on 7 July 1846 when Commo. Jobn D. Sloat sent ashore seamen and Marines from the frigate Savannah and the sloops-of-war Levant and Cyane. Monterey served throughout the Mexican War as the main base of the U. S. squadron on the California coast. It continued that mission until the construction of the Mare Island Navy Yard, now Mare Island Naval Shipyard, Vallejo, Calif., in San Francisco Bay, in 1854.