About that time the U.S. entered into war with Mexico over the far western territories, particularly California. Beale was sent on an urgent errand to Washington D.C. He reported that British ships were amassing on the California coast. Britain was also interested in that territory. Beale was ordered back to his mother ship, the U.S.S. Congress. He sailed the Atlantic, then crossed the Panama overland to meet up with her on the other side. In California, he commanded a contingent of cavalry. Fortunately he was able to subdue the Mexican towns without engaging them in battle. The Mexicans did attempt to take back Los Angeles, and Beale was sent on an important mission to find Captain Fremont in Northern California. When he found them, Fremont's battalion was sailed to Los Angeles to put down the uprising.
Beale was next ordered to escort General Kearney, commander of the Army of the West, who was leading a small force to San Diego. He had come overland after conquering the Mexicans at Santa Fe. His men were tired and their animals were in bad shape. Beale advised Kearney to take the south road to San Diego and avoid confrontation with the Mexicans. However, Kearney was spoiling for a fight and went north. His force was decimated by the Mexicans. Kearney did not plan his tactics well and many could not discharge their weapons after overnight rain dampened their powder. Beale and two others were in charge of the field artillery but unfortunately Kearney had placed it at the back of his force, where it did little good. He sent word to San Diego to send relief, but the messengers were stopped by the Mexicans. They were forced to transport the wounded the Indian way, on travois built by Kit Carson. Beale, Carson, and a Delaware Indian scout finally reached San Diego. Commodore Stockton sent animals and food in a relief column. Beale was promoted to acting lieutenant for his bravery.
