 Most of the settlers at, or near, Sutter's Fort were new immigrants from the United States.Hostilities between the U.S. and Mexico were sparked in part by territorial disputes between Mexico and the Republic of Texas, and later by the American annexation of Texas in 1845.Several battles between U.S. and Mexican troops led the United States Congress to issue a declaration of war against Mexico on May 13, 1846; the Mexican–American War had begun.