On 12 May 1846 the United States declared war on Mexico in a dispute over the boundary between Mexico and the state of Texas, a former Mexican province whose independence and subsequent annexation to the United States Mexico did not recognize. In the Mexican War, which ended with the Treaty of Guadalupe-Hidalgo on 2 February 1848, combined American arms won for the United States the nation’s most decisive victory before the Civil War. The Navy played a major role in securing that victory. By blockading Mexico’s port cities, the Navy strangled Mexico’s maritime trade and prevented its forces from threatening U.S. operations from the sea. The Navy also performed an essential service in transporting men and materiel for the Army. The Navy directed the landing of General Winfield Scott’s troops at Veracruz and participated in the bombardment of that city. By establishing and maintaining sea control, the Navy enabled the Army to seize and garrison enemy territory.
In addition to projecting power against Mexico itself, U.S. naval forces, assisted by a relatively small number of soldiers, seized California for the United States in what was principally a land campaign. In July 1846 Commodore Robert Stockton took over command of the Pacific Squadron and continued the conquest of California begun by Commodore John D. Sloat. In August, Stockton captured Los Angeles, but the following month Mexican Californians expelled the small party of Americans left to garrison the town. In January 1847 Stockton led a force of some six hundred men, with six field pieces, overland to retake Los Angeles. On 8 January he encountered resistance from an organized force of about two hundred armed men. After his men had struggled to push the heavy artillery through the soft bottom and quicksand of a ford of the San Gabriel River, Stockton relied on the field pieces, whose fire he directed personally, to drive off the enemy. The next day, the enemy blocked the way to Los Angeles once more, and once more Stockton used his artillery to disperse the opposition. After these engagements, known as the Battles of San Gabriel and the Mesa, the way to Los Angeles lay open. The Americans reoccupied the town the next day.
The Mexican War experience of Thomas Southwick, carpenter of U.S. frigate Congress, illustrates the crucial roles that the technical skills of essential but unsung warrant officers played in securing victory. Southwick went ashore in California with Commodore Stockton’s force of sailors and marines, attached to an artillery company commanded by navy Lieutenant Richard L. Tilghman. In the Battles of San Gabriel and the Mesa, Southwick had two of the field guns under his charge. Furthermore, the zealous carpenter was instrumental in the general equipping of the artillery. Subsequently, he had charge of a piece of artillery at the capture of Guaymas, a Mexican port in the Gulf of California; and in the attack on Mazatlán, a Mexican coastal city near the mouth of the Gulf of California, he landed with the attacking party, again in charge of a piece of artillery. During the seven-month occupation of Mazatlán, Southwick served ashore where he aided in the engineering and construction of fortifications and the fabrication of gun carriages and in addition had charge of one of the forts.
Securing for the United States not only Texas but also New Mexico Territory and California, the Mexican War left the nation with two sea coasts to defend, propelled the United States into Pacific affairs, and provided impetus for the Navy’s expansion. The war also left a body of tactical experience on which officers in the Union and Confederate navies would draw during the Civil War.