As Tyler assumed the presidency after Harrisons
death, the United States became engaged in a series of sharp disputes with
Britain. A conflict over the Maine boundary was resolved, but British involvement
in Texas revived the movement to annex the Lone Star Republic to the United
The Texas and Oregon questions became embroiled
in the 1844 campaign, as the Democrats nominated and elected the militantly
expansionist Polk. After Texas was added to the Union, conflicts with Mexico
over California and the Texas boundary erupted into war in 1846.
American forces quickly conquered California
and New Mexico. Winfield Scotts and Zachary Taylors invasion
of Mexico was also successful, and the United States obtained large new territories
in the peace treaty.
Besides adding California, New Mexico, and Utah
to American territory, the Mexican War trained a new generation of military
leaders and aroused long-term Latin American resentment of the United States.
Most important, it forced the slavery controversy to the center of national
debate, as first indicated by the Wilmot Proviso.