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Mexican–American War

During the Mexican–American War, the desertion rate in the U.S. Army was 8.3% (9,200 out of 111,000), compared to 12.7% during the War of 1812 and usual peacetime rates of about 14.8% per year. Many men deserted in order to join another U.S. unit and get a second enlistment bonus. Others deserted because of the miserable conditions in camp, or in 1849–1850 were using the army to get free transportation to California, where they deserted to join the California Gold Rush. Several hundred deserters went over to the Mexican side; nearly all were recent immigrants from Europe with weak ties to the United States. The most famous group was the Saint Patrick's Battalion, about half of whom were Catholics from Ireland, anti-Catholic prejudice reportedly being another reason for desertion. The Mexicans issued broadsides and leaflets enticing U.S. soldiers with promises of money, land grants, and officers' commissions. Mexican guerrillas shadowed the U.S. Army, and captured men who took unauthorized leave or fell out of the ranks. The guerrillas coerced these men to join the Mexican ranks—threatening to kill them if they failed to comply. The generous promises proved illusory for most deserters, who risked execution if captured by U.S. forces. About fifty of the San Patricios were tried and hanged following their capture at Churubusco in August 1847.

High desertion rates were a major problem for the Mexican army, depleting forces on the eve of battle. Most of the soldiers were peasants who had a loyalty to their village and family but not to the generals who conscripted them. Often hungry and ill, never well paid, under-equipped and only partially trained, the soldiers were held in contempt by their officers and had little reason to fight the Americans. Looking for their opportunity, many slipped away from camp to find their way back to their home village.