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 States.

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.