The slave-owning nation of the United States attacked the free nation of Mexico on April 25th, 1846, ultimately annexing most of it and later declaring Mexicans to be somehow foreign. As the war dragged on, thousands of US troops deserted from the ranks of the Army. 202 of them — mostly Irish but including various other nationalities, such as Poles — took desertion a big step further, and formed the Mexican Army’s only foreign legion, the St Patrick Battalion.