Saint Patrick's Battalion, a unit of hundred immigrants of European descents, joined Mexican Army and fought against United States in Mexican-American War from 1846 to 1848. Why did those immigrant soldiers desert U.S. and assist their enemies? The success of United States invasion and control of vast territories of Mexico must rely on soldiers' unity. However, the desertion of immigrant soldiers revealed some sharp conflicts among U.S. Army. U.S. Army's prejudice to those immigrant soldiers pushed them to desert. Soldiers' desertion was very rare in the war history; thus, the main reason of religious discrimination, poor living condition and inhuman treatment by U.S. Army and temptation by Mexican government will be explored in this paper to explain why those immigrant deserters gave up United States and changed their side in Mexican-American War.
            
             Many immigrant soldiers, especially Irish, deserted because they experienced harsh religious discrimination in the United States. Because of the difficult condition of famine in Ireland, many Irish immigrants went to United States. Thousands of them joined U.S. Army for the U.S. Citizenship and money. However, the tension rose between Army and Irish due to the religion. Most of Irish were Catholic and U.S. Army was very intolerant towards both Irish and Catholics at that time. In Peter F. Stevens's "A Perilous Journey and Six Irish Rebels' Escape to Freedom", he wrote the prejudices towards Irish in the U.S. Army, "Irish were seen as lazy and ignorant, while Catholics were considered fools who were easily distracted by pageantry and led by a faraway pope"(Stevens). In the army, other soldiers believed that the Irish were inferior soldiers and gave them dirty jobs. They had no chance to be promoted. Punishments such as negligence of duty were often severe. Even worse, conditions were harsh for other Catholic immigrant soldiers such as the German....