The boundary of the new state was ill-defined. Texas claimed to the Rio Grande. Mexico would probably have acknowledged the Nueces River. The United States attempted to enforce the claim of Texas, sent troops to the Rio Grande, and so brought on the Mexican war.
At the close of the Mexican war the boundary of the United States was carried south from forty-two degrees to the Gila River, and what is now California, Nevada, Arizona, New Mexico, Utah, and more than half of Wyoming and Colorado were added to the public domain. While the war was still raging, Polk, who had succeeded Tyler, asked for two million dollars to aid him in negotiating peace. Well knowing that the money was to be used to buy land from Mexico, David Wilmot moved in the House of Representatives that from all territory bought with the money slavery should be excluded. This was the famous Wilmot proviso. It failed of adoption and the territory was acquired in 1848, with its character as to slavery or freedom wholly undetermined.
The Preliminary Struggle over the Slave Problem.—And now the old parties began to break up. Democrats who believed in the Wilmot proviso, and Whigs who detested the annexation of Texas, the war with Mexico, and the extension of slavery went over in a body to the Liberty Party, formed with it the Freesoil Party, nominated Martin Van Buren, and gave him three hundred thousand votes. In their platform they declared that Congress had no more power to make a slave than to make a king; that they accepted the issue thrust on them by the South; that to the demand for more slave states and more slave territories they answered, no more slave states, no more slave territories; and that on their banner was inscribed “Free Soil, Free Speech, Free Labor, and Free Men.” As the defection of Whigs to the Liberty Party in 1844 gave New York state to the Democrats and elected Polk, so the defection of Democrats to the Free Soilers in 1848 gave New York to the Whigs and elected Taylor. As Harrison, the first Whig president, died one month after taking office, so Taylor, the second Whig president, died suddenly when a little over one year in office, just as the great Whig compromise of 1850 was closing. The imperative need of civil government in the new territory, the discovery of gold in California, the rush of men from all parts of the earth to the Pacific Coast forced Congress to establish organized territories. The question was: shall they be opened or closed to slavery? But, as the soil had been free when acquired from Mexico, the question really was: shall the United States establish slavery?
THE MEXICAN WAR, 1846-1848
Causes of the War
(1) Long-standing irritation over claims of American citizens upon Mexico, which the latter refused to pay.
(2) The annexation by the United States of Texas, which Mexico claimed as still a part of her territory.
(3) Disputes as to whether the Rio Grande or Nueces River was the boundary of Texas.
Results of the War