Army of the West ( 1846 ) The Army of the West was the name of the United States force commanded by Stephen W . Kearny during the Mexican - American War , which played a prominent role in the conquest of New Mexico and California . It was headquartered at Fort Leavenworth , Kansas . After the war , it was broken up into the Department of the Pacific and the Department of the West . New Mexico . Colonel Stephen W . Kearny initially commanded some 1 , 700 regular army and volunteer soldiers mustering at Fort Leavenworth , Kansas . Kearny was promoted to brigadier general , and he designated the force the " Army of the West " and advanced on the Santa Fe trail by the end of June 1846 . Other American forces followed including Colonel Sterling Price and the Second Missouri Mounted Volunteer Regiment and the famous Mormon Battalion the only religious unit in American military history . Months later the 1st New York Volunteer Infantry and some regular army units arrived by ship in California . Kearny moved into present day New Mexico and seized Santa Fe , establishing a military government there . Kearny dispatched forces under Colonel Alexander Doniphan for further operations in New Mexico while Kearny with the remaining forces of the army proceeded overland towards California , guided by Kit Carson . Colonel Sterling Price was left in command of U . S . troops in New Mexico . The forces left behind in New Mexico were still regarded as part of " Army of the West " . Price was able to successfully put down the Taos Revolt in the early part of 1847 . Eventually the Army of the West would have some 4 - 5 , 000 soldiers operating in California , New Mexico and also Doniphan ' s regiment and its expedition in Chihuahua . California . With just under 200 troops Kearny moved into the Alta California Province of Mexico and hastily attacked a Californio force at the Battle of San Pasqual . The battle was costly to the U . S . , and Kearny moved on and occupied San Diego where he was joined by naval reinforcements serving as infantry under Commodore Robert F . Stockton . The combined force numbering 600 dragoon s and sailors led by Kearny and Stockton attacked the Californios at Los Angeles . Winning the battles of Rio San Gabriel and La Mesa , the U . S . forces took control of the city on January 10 , 1847 . On January 13 , the Californios surrendered to John C . Fremont , and the major fighting in California was over . Chihuahua . In late 1846 , General Stephen W . Kearney headed west to California , where he was soundly defeated in several skirmishes and at the Battle of San Pasqual by the far superior Californio Lancers . He left three companies of Dragoons to form , with volunteer troops , a strong garrison in New Mexico . Command of the garrison eventually was placed with Colonel Sterling Price . The last major action of the army came in 1848 when Price , recently promoted to the rank of general , invaded the state of Chihuahua and defeated a Mexican force at the Battle of Santa Cruz de Rosales . This battle occurred after a cease fire and the Treaty of Guadalupe Hidalgo had been signed . In 1862 , Sterling Price would command another force named Army of the West , this time fighting against United States during the American Civil War . Order of Battle at Santa Cruz de Rosales