Following the declaration of war against Mexico in May 1845, efforts were quickly undertaken to gain control of the remote Mexican provinces of New Mexico and California.
General Stephen W. Kearny was dispatched from Fort Leavenworth, Kansas, and marched over the Santa Fe Trail into the Southwest. He seized Las Vegas, Taos and Santa Fe (August 18, 1846) without opposition.
Kearny established a new government in New Mexico, then divided his small force into three even smaller parts:
Kearny headed one contingent and began a march westward to seize California. Along the trail, he met Kit Carson who erroneously informed him that California had already fallen into American hands. Most of Kearny's soldiers were then returned to New Mexico
Alexander W. Doniphan had the difficult assignment of moving south into Mexico to capture the city of Chihuahua. His force experienced many hardships, but successfully gained its objective on February 28, 1847. Doniphan later moved eastward and linked his force with Zachary Taylor near Saltillo.
- Sterling Price was assigned the task of maintaining control in New Mexico. Considerable resistance was put up by Native Americans, who opposed the presence of Mexicans or Americans. It took Price months to pacify the area.
American squatters in California, anxious to be rid of Mexican control, staged a largely bloodless revolt and established the Bear Flag Republic in Sonoma in June 1846. In July, Commodore John D. Sloat secured Monterey and San Francisco, then proclaimed California to be under American control. However, fighting against independent-minded californios, which involved the combined efforts of Kearny, Stockton and John C. Frémont, continued into January 1847.