American President James K. Polk intended to invade and hold the territories he wanted, so he sent General Stephen Kearny west from Fort Leavenworth with 1,700 men to invade and hold New Mexico and California. Kearny captured Santa Fe and then divided his forces, sending a large contingent south under Alexander Doniphan. Doniphan would eventually take the city of Chihuahua.
Meanwhile, the war had already begun in California. Captain John C. Fremont had been in the region with 60 men: they organized American settlers in California to revolt against the Mexican authorities there. He had the support of some US navy vessels in the area. The struggle between these men and the Mexicans