Thornton Affair — First Blood of the War

A Mexican cavalry force of approximately 1,600 men crossed the Rio Grande and ambushed a 63-man US dragoon patrol under Captain Seth Thornton north of the river near Rancho de Carricitos. Eleven Americans were killed, five wounded, and 49 captured. Polk used this clash to tell Congress: 'Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil.' Critics including Rep. Abraham Lincoln disputed that the site was 'American soil.'