<P> In 1845, newly elected U.S. President James K. Polk made a proposition to the Mexican government to purchase the disputed lands between the Nueces River and the Rio Grande river further south . When that offer was rejected, President Polk moved U.S. troops commanded by Major General Zachary Taylor further south into the disputed territory . Mexican forces attacked an American Army outpost ("Thornton Affair") in the occupied territory, killing 12 U.S. soldiers and capturing 52 . These same Mexican troops later laid siege to an American fort along the Rio Grande . Polk cited this attack as an invasion of U.S. territory and requested that the Congress declare war . </P> <P> U.S. forces quickly occupied the capital town of Santa Fe de Nuevo México along the upper Rio Grande and the Pacific coast territory province of Alta California (Upper California). They then invaded to the south into parts of central Mexico (modern - day northeastern Mexico and northwest Mexico). Meanwhile, the Pacific Squadron of the United States Navy conducted a blockade and took control of several garrisons on the Pacific coast farther south in lower Baja California Territory . The U.S. Army, under the command of Major General Winfield Scott, after several fierce battles of stiff resistance from the Mexican Army outside of the capital, Mexico City, eventually captured the city, having marched west from the port of Veracruz, where the Americans staged their first amphibious landing on the Gulf of Mexico coast . </P> <P> The 1848 Treaty of Guadalupe Hidalgo, forced onto the remnant Mexican government, ended the war and specified its major consequence, the Mexican Cession of the northern territories of Alta California and Santa Fe de Nuevo México to the United States . The U.S. agreed to pay $15 million compensation for the physical damage of the war . In addition, the United States assumed $3.25 million of debt already owed earlier by the Mexican government to U.S. citizens . Mexico acknowledged the loss of their province, later the Republic of Texas (and now the State of Texas), and thereafter cited and acknowledged the Rio Grande as its future northern national border with the United States . Mexico had lost over one - third of its original territory from its 1821 independence . </P> <P> The territorial expansion of the United States toward the Pacific coast had been the goal of Polk, the leader of the Democratic Party . At first, the war was highly controversial in the United States, with the Whig Party, anti-imperialists, and anti-slavery elements strongly opposing . Critics in the United States pointed to the heavy casualties suffered by U.S. forces compared to earlier American wars, and the conflict's high monetary cost . The war intensified the debate over slavery in the United States, contributing to bitter debates that culminated in the American Civil War (1861--1865). </P>

The land gained by the united states as a result of the mexican american war is known as the