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".

[6] General Kearny's Army, most originating from Fort Scott:[12][13][14][15] Captains Johnston, Griffin (Surgeon),[30] and Turner[31] kept journals during their journey from Santa Fe.

[32] Lieutenant W. H. Emory of the Topographical Engineers kept the official designated U.S. Government diary, or "Military Reconnoissance" [sic] from Ft. Leavenworth to California which was published in 1848.

[16] In a coincidence that has never been fully explained, a guard under the command of Machado's concuñado, the brother of a brother-in-law and future father-in-law, Captain Jose Alipaz, challenged the dragoons and alerted the camp to their presence.

[16] While Machado quickly ran back to Hammond's scouting party, Alipaz sounded the alarm but was dismissed by General Pico, until a U.S. Army blanket and dragoon coat were discovered on the edge of camp by Pablo Véjar.

Alipaz, Captain Leonardo Cota and José María Ibarra (the Californio standing guard) chased the dragoons to the top of the next ridge with the battle cry of "!Viva California!".

[7]: 187 [48][49] Kearny had planned a surprise attack at daylight, despite the damp weather wetting down their powder and the extremely poor state of the soldiers' equipment and mounts – mostly mules, as the horses had died on the preceding march.

Early in the morning of December 6, 1846, the column proceeded by twos across the ridge between Santa Maria (present day Ramona, California) and San Pasqual.

During the descent, while it was still dark and with a low-lying fog, Kearny's force became strung out and were caught in a disadvantageous position by General Pico's swift advance.

[39] Kearny gave the plan of battle prior to proceeding down into the valley, to keep all casualties to a minimum, to encircle San Pasqual to capture fresh mounts.

[6] Forty of the best mounted pulled far ahead of the main body of the force, in violation of the Cavalry Tactics manual of 1841, which instructed a charge to begin at just 40 paces from the enemy "so as to arrive in good order, and without fatiguing the horses.

Damp powder reduced the effectiveness of carbines to clubs and pistols to hammers, as described by Felicita[51] a San Pasqual Indian[52] that witnessed the battle.

Gillespie used a sabre to fight off a vicious personal attack made by a group of lancers in revenge for his previous actions during his occupation of Los Angeles and the broken agreement to cease hostilities.

[6] Gillespie's men unlimbered the remaining howitzer – John Sutter's Russian-made bronze four-pounder – and were able to drive the Californio fighters from the field after Midshipman Duncan fired canister into them.

Summarizing the battle, historian Owen Coy writes: The Americans fought bravely against heavy odds, for their mules were unmanageable, and their sabers too short to cope effectively with the long California lances.

[55]The next day, December 7, 1846, after assurances by Dr. Griffin that the worst of the injured could be moved, Captain Turner, now in command of the Dragoons, since Kearny was wounded, marched the column toward San Diego.

[58] That evening, Captain Turner, established a strong defensive perimeter and then sent dispatches requesting urgent reinforcements, carried to Commodore Stockton by "Alexis Godey, Thomas Burgess, and one other.

Prior to capture by the Californios on their return trip, they had hid Stockton's message under a tree, "but when this cache was examined the letters were missing.

Beale, Scout Carson, and an Indian guide named Pontho moved[62][63][64][65] under cover of darkness, taking different routes to the commodore's headquarters at San Diego, 28 miles (45 km) to the south-southwest.

But "to the young naval officer it was pure torture, and upon his arrival at old town [San Diego], being unable to stand because of his lacerated feet, he had to be carried into Commodore Stockton's headquarters."

[58] Dr. John S. Griffin, Kearny's surgeon, reported that the Americans had lost 17 killed and 18 wounded out of the 50 officers and men who engaged Pico's lancers.

[9] Historian Arthur Woodward wrote, "One can only suppose that Kearny, having made one of the longest marches in the history of the United States Army, was spoiling for a fight and intended to have it.

The following day the Battle of La Mesa resulted in another Californio defeat, leading to the surrender of the Pueblo de Los Ángeles and later the signing of the Treaty of Cahuenga.

While Kearny made a poor decision to engage the Californios at San Pasqual, the operations on either side of the battle revealed a brilliant military mind coordinating complex actions across the expanse of a continent.