Stephen Watts Kearny was one of the foremost antebellum frontier officers of the United States Army. He is remembered for his significant contributions in the Mexican–American War, especially the conquest of California. The Kearny code, proclaimed on September 22, 1846, in Santa Fe, established the law and government of the newly acquired territory of New Mexico and was named after him. His nephew was Major General Philip Kearny of American Civil War fame.

The Mexican–American War, also known in the United States as the Mexican War, and in Mexico as the United States intervention in Mexico, was an invasion of Mexico by the United States Army from 1846 to 1848. It followed the 1845 American annexation of Texas, which Mexico still considered its territory because Mexico refused to recognize the Treaties of Velasco, signed by President Antonio López de Santa Anna after he was captured by the Texian Army during the 1836 Texas Revolution. The Republic of Texas was de facto an independent country, but most of its Anglo-American citizens who had moved from the United States to Texas after 1822 wanted to be annexed by the United States.