The Pacific Coast Campaign refers to United States naval operations against targets along Mexico's Pacific Coast during the Mexican–American War. It excludes engagements of the California Campaign in areas of The Californias north of the Baja California Peninsula. The objective of the campaign was to secure the Baja Peninsula of Mexico, and to blockade/capture west-coast ports of Mexico—especially Mazatlan, a major port-of-entry for imported supplies. The resistance of Mexican forces to the north in the Los Angeles area and the lack of ships, soldiers and logistical support prevented an early occupation of the peninsula and the west-coast Mexican seaports. The U.S. Navy attempted blockades of the ports three times before being able to successfully blockade and/or occupy them.
Following an easy initial occupation and the capitulation of La Paz by Governor Col. Francisco Palacios Miranda, loyalist residents met, declared Miranda a traitor, and rose in revolt. Under a new governor, Mauricio Castro Cota, and then under the leadership of Manuel Pineda Munoz (who defended Mulege from American landings), the loyalists attempted to expel the Americans from La Paz and San José del Cabo. Pineda was eventually captured and the Mexican army under Cota finally defeated at Todos Santos but only after the Treaty of Guadalupe Hidalgo that ended the war returned captured regions south of San Diego to Mexico.
First Blockade of the West Coast of Mexico
Following the July–August 1846 campaign led by the Commander of the U.S. Navy Pacific Squadron, Commodore Robert F. Stockton proclaimed United States control of northern regions of Las Californias (known separately before 1836 as Alta California), on August 17, 1846. On August 19, Commander Stockton ordered Joseph B. Hull, commander of the USS Warren, to blockade Mazatlan. Samuel F. Dupont, commander of the second class sloop-of-war Cyane, was ordered to blockade San Blas (about 125 miles south of Mazatlan). Stockton intended to seize Acapulco, and use it as a base for a joint Army-Navy expedition into Mexico. On September 2, 1846 the Cyane captured two Mexican vessels in the harbor then a landing party spiked 34 cannons in the port of San Blas. On September 7, the Warren seized the Mexican brig Malek Adhel at Mazatlan.
In an August 17 proclamation, Stockton had claimed United States control of the Baja Peninsula. To make good on this claim, Commander Dupont then sailed north to La Paz (on the Gulf of California), seized nine small pearl-fishing boats and secured a promise of neutrality from Colonel Francisco Palacios Miranda, governor of Baja California. On October 1, the Cyane seized two schooners at Loreto (about 150 miles north of La Paz). On October 7, the Cyane shelled Guaymas on the mainland after Colonel Antonio Campazano refused to surrender it. A boarding party from the Cyane seized the brig Condor in that port but, finding it unusable, burned it.
The revolt of Californians in Los Angeles (September 27, 1846) prevented the resupply and replacement of the blockade force who could not maintain station without them. When news of the revolt came the Warren left for San Francisco. On November 13, the Cyane followed ending the first blockade of Mexico's west coast after about four weeks.
Second Blockade of the West Coast of Mexico, Occupation of Baja California Sur
On December 24, 1846, Secretary of the Navy John Y. Mason ordered Stockton to impose an effective blockade on the west coast of Mexico. Its object was to prevent the enemy from getting munitions and other supplies and to make possible the landing of American soldiers.
The Battle of La Mesa January 9, 1847 was the last armed resistance to the United States conquest of California. The conquest and annexation of was confirmed with the signing of the Treaty of Cahuenga by US Army Lieutenant-Colonel John C. Frémont and Mexican General Andrés Pico on January 13, 1847. With the fate of California settled, on January 11, 1847, Secretary of War William L. Marcy instructed General Stephen Watts Kearny, commanding the Army forces in California, to make the American hold on the Californias so secure that none could successfully challenge it. However, at the time Kearny had only one company of troops (about 100 men), and awaited reinforcements.