Following a clash of U.S. forces with Mexican forces near the Rio Grande, Colonel Stephen Watts Kearny was promoted to a brigadier general and tasked with multiple objectives to include the seizure of New Mexico and California, establish civilian government within seized territories, disrupt trade, and to "act in such a manner as best to conciliate the inhabitants, and render them friendly to the United States". Kearny's initial force consisted of 300 regular army soldiers, 1,000 volunteers from Missouri, and the Mormon Battalion. From Fort Leavenworth, via Bent's Fort, Kearny had New Mexico capitulate without violent conflict. While in Sante Fe, Kearny established Fort Marcy, named after the Secretary of War William L. Marcy, who had ordered Kearny's force westward.
En route from New Mexico, Kearny's force interacted with the Apache and Maricopa tribes, and captured a Mexican courier with news of American activities in California, with the news stating the Californios had capitulated. Forces under Commodore Sloat had taken control a significant portion of Alta California. Kearny had orders to assume command of U.S. forces in California with his, but sent back most of his force after meeting up with Kit Carson near Socorro on 6 Oct. and hearing of the seizure of California by Commodore Robert F. Stockton, Kearny keeping only Companies C & K, 1st Dragoons, about 100 men.:137 Kearny, at that time with a force of 300 men, learning of escalating issues with the Navajo, and with the belief a smaller force could move faster, ordered 200 back to Santa Fe. Kearny's force, guided by Carson, reached Warner's Ranch in California on 2 Dec., in a greatly weakened condition.:187 They had just completed a 2,000 mile march; the longest march in U.S. Army history; the force was travel weary and mounted mules and half-broken horses which were rounded up around Warner Ranch that were owned by California Capt. Jose Maria Flores.
General Kearny's Army, most originating from Fort Scott:
- Captain Abraham Robinson Johnston – regimental adjutant, Company K, 12 mounted dragoons
- Captain Benjamin (Ben) Daviess Moore – Company C, 60 dismounted dragoons, some mounted on mules
- Captain Henry Smith Turner – Kearny's Army of the West Adjutant general
- Lieutenant William H. Emory – Chief Topographical Engineer, Corps of Topographical Engineers
- Lieutenant William H. Warner – Corps of Topographical Engineers, commanding four topographical engineering 'mountainmen' Peterson, Londeau, Perrot, and Private Francois Menard
- Lieutenant John W. Davidson – commanded 2 howitzers and 6 dragoons placed at the rear of the advance
- Second Lieutenant Thomas (Tom) C. Hammond – aide-de-camp 
- Major Swords – assistant quartermaster – rear guard for baggage train, officers' personal slaves, and civilians
- U.S. Army Surgeon (Captain) Dr. John S. Griffin
- Enlisted men:
- Judge Pearce (Kearny's personal bodyguard), Sergeant Williams, Pat Halpin (bugler), Sergeant Falls, Sergeant John Cox, Private William B. Dunne, Private David Streeter, Private James Osbourne, (Private) Dr. Erasmus Darwin French(physician asst.)
- Company C: Corporal William C. West, Private George Ashmead, Private Joseph T. Campbell, Private John Dunlop, Private William Dalton, Private William C. Leckey, Private Samuel F. Repoll, Private Joseph B. Kennedy,
- Company K: 1st Sergeant Otis L. Moor, Sergeant William Whitness, Corporal George Ramsdale, farrier David W. Johnston, Private William G. Gholston, Private William H. Fiel, Private Robert S. Gregory, Private Hugh McKaffray
Both Emory and Johnston kept journals during their journey from Santa Fe.
After turning back the Americans trying to recapture Los Angeles in the Battle of Dominguez Rancho, Capt. Jose Maria Flores sent about 100 men to San Luis Obispo to confront Lt. Col. John C. Fremont's 300 men moving south from Monterey, and sent another 100 men to watch Stockton's base at San Diego, but Flores kept the bulk of his men at Los Angeles.:186
Captain Archibald Gillespie with 39 men, met Kearny on 5 Dec. with a message from Stockton requesting Kearny confront Flores' men outside San Diego.:187 The total American force now amounted to 179 men.
USMC Acting-Captain (Lt.) Gillespie's Mounted Rifle Volunteers 'detachment' of the California Battalion:
- 10 U.S. Navy sailor carbiners from 'Fleet on the Pacific Coast' F Company aboard the USS Congress commanded by Acting-Lieutenant Edward Fitzgerald Beale and Passed-Midshipman James M. Duncan (later commanded USS Norwich (1861) )
- Sutter Fort's Russian brass 4-pounder cannon; it had been hidden after Mexican authorities tried to use the cannon against Californios at the Battle of Cahuenga Pass during the 1831 tax revolt. Charles Weber disclosed its location to Gillespie.
- 26 Mounted Rifle Company, commanded by Acting-Captain (Sgt.) Samual Gibson (later commanded company B of 26th Arkansas Infantry Regiment) and longtime Kit Carson and John Fremont associate, Acting-Lieutenant Alexander Godey: including Antoine Robidoux(interpreter), Philip Crosthwaite, Beatitude Patitoux, William (Col. Owl) Henry Russel l, Daniel Sexton, Franklin Sears, Thomas Burgess, Jean Nutrelle, Private Henry Booker.
- Rafael Machado, Californio guide provided by San Diego
Captains Leonardo Cota and Jose Alipaz took a force to San Pasqual Valley with the intention to interdict and keep in check Captain Gillespie after his departure from San Diego. Later, Major Andrés Pico, after a failed search for a detachment of U.S. soldiers, joined forces with the captains and took command. These Californios led a force consisting of landowners, sons of landowners, and vaqueros, many with well known and respected family names in the community:
- Don Leonardo Cota: Capt. Enrique Abilia (Los Angeles), Capt. Ramon Carillo(Los Angeles), Capt. Jose Maria Cota(Los Angeles), Capt. Carlos Dominguez(Los Angeles), Capt. Nicolas Hermosillo(Los Angeles), Capt. Jose Alipaz (San Juan Capistrano), Capt. Ramon O. Suna (San Diego)
- General Andres Pico: Don Leandro Osuna (San Diego), Capt.
Juan Bautista Moreno, Capt.
Tomás A. Sanchez, Capt.
Pablo Vejar, Capt. Manuel Vejar
On the night of the 5th, a Native American informed the Californio forces of the presence of Kearny's forces.