In 1846, the United States went to war against Mexico and sent a naval and land expedition to conquer Alta California. At first they had an easy time of it capturing the major ports including San Diego, but the Californios in southern Alta California struck back. Following the successful revolt in Los Angeles, the American garrison at San Diego was driven out without firing a shot in early October 1846. Mexican partisans held San Diego for three weeks until October 24, 1846, when the Americans recaptured it. For the next several months the Americans were blockaded inside the pueblo. Skirmishes occurred daily and snipers shot into the town every night. The Californios drove cattle away from the pueblo hoping to starve the Americans and their Californio supporters out. On December 1 the Americans garrison learned that the dragoons of General Stephen W. Kearney were at Warner's Ranch. Commodore Robert F. Stockton sent a mounted force of fifty under Captain Archibald Gillespie to march north to meet him. Their joint command of 150 men, returning to San Diego, encountered about 93 Californios under Andrés Pico. In the ensuing Battle of San Pasqual, fought in the San Pasqual Valley which is now part of the city of San Diego, the Americans suffered their worst losses in the campaign. Subsequently a column led by Lieutenant Gray arrived from San Diego, rescuing Kearny's battered and blockaded command.

Who commissioned Captain Archibald Gillespie to lead 50 men on horseback to the north?