By 1845, the United States had grown to twenty-seven states and several territories, standing on the brink of massive territorial expansion that would extend the nation to the Pacific Ocean.

The nation had added four states since 1820: Missouri (1821), Arkansas (1836), Michigan (1837), and Florida became a state in March 1845. Texas was poised for annexation and would join as the twenty-eighth state in December 1845. The Missouri Compromise continued governing the balance between free and slave states, though tensions were mounting over western expansion.

The Iowa Territory was approaching statehood requirements and would be admitted in 1846. The Wisconsin Territory, carved from Michigan Territory, included present-day Wisconsin, Minnesota, and parts of the Dakotas. The vast unorganized territory from the Louisiana Purchase remained largely ungoverned, extending to the Rocky Mountains and beyond to the Oregon Country.

The Republic of Texas, independent since 1836, dominated territorial discussions in 1845. Texas annexation was completed through a joint congressional resolution rather than treaty, avoiding the two-thirds Senate majority required for treaties. This acquisition added approximately 389,000 square miles and intensified sectional tensions over slavery's expansion.

The Oregon Country remained under joint British-American occupation, with the "54°40' or Fight!" slogan reflecting American territorial ambitions. Mexican control over California, New Mexico, and the Southwest was increasingly tenuous as American settlers moved westward and tensions with Mexico escalated over Texas boundaries.

Territorial governance faced unprecedented challenges managing vast distances and diverse populations. The federal government struggled to establish effective administration across such enormous territories while balancing sectional interests.

The concept of Manifest Destiny was driving this territorial expansion, with Americans increasingly viewing continental expansion as their national destiny, setting the stage for the Mexican-American War and massive territorial acquisitions.