Between 1845 and 1848 the United States expanded its territory over 1 million square miles. This was more land than was acquired in the Louisiana Purchase. The government gained this huge amount of territory through a variety of means—annexation, peaceful diplomacy, and war.

The United States annexed Texas in 1845. Texas had already won its independence from Mexico in 1836 during a series of battles during the Texas revolution. Texas thereby became an independent state. It then began petitioning to become part of the United States. The question of whether to admit Texas to the Union was debated by Congress and the White House for ten years before President James K. Polk, who had run on an expansionist ticket, signed the proclamation making Texas the twenty-eighth state.

During the 1840s American pioneers were beginning to migrate to the choice farming country in Oregon. This area was formally claimed by Britain...