While these revolutionary movements were destroying the power of Mexico in the interior of the Province of California, and the expedition under General Kearney--ignorant of the fact that the work had been done already--was approaching its eastern borders for the same purpose, the naval force of the United States in the Pacific, under Commodore Sloat, had been assisting in the work of conquest. Having heard of the opening of hostilities on the Rio Grande, the Commodore--then at Mazatlan--hastened with the Savannah to Monterey in California, where he arrived on July 2d, and on the 7th he took possession of the town without opposition; the custom-house was seized, the American flag raised, and California declared to be "henceforward a part of the United States." Within a few days intelligence of the action of Commodore Sloat was received by the revolutionary leaders at Sonoma; and a battalion of mounted riflemen which had been organized among them was immediately moved to Monterey, the flag of the United States was substituted for the "bear and star," and the authority of the Commodore was immediately recognized. This battalion of mounted riflemen on its arrival at Monterey, July 23, 1846, was mustered into the service of the United States by Commodore Stockton, who had succeeded Commodore Sloat in command of the squadron--Captain Frémont being appointed its commandant, and Lieutenant A. H. Gillespie, of the Marines, its second officer--and it was immediately despatched on the sloop-of-war Cyane to San Diego for the purpose of cutting off the retreat of General Castro, of the Mexican service, who had encamped and fortified his position near Ciudad de los Angeles, while the Commodore with his sailors--who landed from the Congress at San Pedro--moved against him in front. The expedition was eminently successful, as the Mexicans on the approach of the Commodore immediately evacuated their camp and fled in the greatest confusion--although most of the principal officers were subsequently captured--and, on August 13th, the Ciudad de los Angeles was occupied, again without opposition, by the American troops and seamen, and the conquest of California was apparently completed. A short time afterward Commodore Stockton appointed Captain Frémont Governor of the Territory into which, by the proclamation of Commodore Sloat, the Province had been transformed; while Captain Gillespie was left, with nineteen men, in possession of Los Angeles; Lieutenant Talbot, of the Topographical Engineers, with nine men, was left at Santa Barbara; and, with his squadron, Commodore Stockton proceeded to San Francisco; while Governor Frémont, on September 8th, also moved to Monterey.