MEXICAN WAR DECLARED: 175th ANNIVERSARY. May 13, 1846. Although fighting had begun days earlier, Congress officially declared war on Mexico on this date. The struggle cost the lives of more than 13,000 Americans and 25,000 Mexicans and resulted in the annexation by the US of land that became parts of Oklahoma, New Mexico, Arizona, Nevada, California, Utah and Colorado. The war ended in 1848.
