On April 25, 1846, the United States declared war with the Republic of Mexico after a long dispute over the U.S. annexation of Texas. General Stephen Watts Kearny’s Army of the West marched out of Fort Leavenworth on June 28, 1846 with orders to conquer New Mexico and California, and prosecute the war into Mexico. Lieutenant William Hemsley Emory, along with a small staff of topographers and scientists, was detailed to study and map the route.
They occupied Santa Fe on August 18 without a fight and set up a military government in New Mexico. In early September Kearny took 300 soldiers, the topographers, wagons, howitzers, and cattle down the Rio Grande River headed for California. The rest of the army was split, part of them assigned to occupy and defend New Mexico and the rest sent into Mexico to continue the war there. Near Socorro Kearny met Kit Carson who was taking news to Washington that California had surrendered to Colonel Fremont and Commodore Stockton. On hearing this, Kearny decided to send all but 100 soldiers back to Santa Fe and ordered Carson to guide him back along the Gila Trail. Kearny’s first guide, Thomas Fitzpatrick, was sent on to Washington with the news. Carson told Kearny that, the journey through the canyons of the Gila would be extremely slow and difficult for wagons so he sent back the wagons, keeping only an instrument wagon and the small howitzers, and transferred their supplies to pack mules.
Carson led the army across southern New Mexico and down the Gila River to its mouth. They crossed the Colorado River near the future town of Yuma and pressed on through the hot and dry Imperial Desert of southern California and northern Mexico. At San Pasqual in California, the army fought its first battle with the Mexicans. The Mexicans inflicted severe casualties and the army only escaped complete defeat because of the heroic actions of Kit Carson, Edward Beale, and an Indian, who sneaked through enemy lines at night to get reinforcements from San Diego. The army reached San Diego on December 12, 1846.
Lieutenant Emory’s official report of the expedition (Notes of a Military Reconnoissance From Fort Leavenworth in Missouri to San Diego, in California) was so well received that it went through many printings and editions.