After the declaration of war on May 13, 1846, United States Army General Stephen W. Kearny moved southwest from Fort Leavenworth, Kansas, in June 1846 with about 1,700 men in his Army of the West. Kearny's orders were to secure the territories Nuevo México and Alta California.

In Santa Fe, Governor Manuel Armijo wanted to avoid battle, but on August 9, Colonel Diego Archuleta and militia officers Manuel Chaves and Miguel Pino forced him to muster a defense. Armijo set up a position in Apache Canyon, a narrow pass about 10 miles (16 km) southeast of the city. However, on August 14, before the American army was even in view, he decided not to fight. The New Mexican army retreated to Santa Fe, and Armijo fled to Chihuahua.

Kearny and his troops encountered no Mexican forces when they arrived on August 15. Kearny and his force entered Santa Fe and claimed the New Mexico Territory for the United States without a shot fired. Kearny declared himself the military governor of the New Mexico Territory on August 18 and established a civilian government. American officers drew up a temporary legal system for the territory called the Kearny Code.