At independence the thirteen states hugged the Atlantic coast. Victory in the Revolutionary War extended the nation to the Mississippi by the Treaty of Paris (1783), about 2.3 million km² in all, and the Louisiana Purchase (1803) doubled that territory, laying the foundation for continental expansion.

Through westward expansion, war and settlement the United States reached the Pacific, forming a coast-to-coast nation of about 8.1 million km² after the Gadsden Purchase of 1853. Key events included the annexation of Texas, the Mexican–American War and the California Gold Rush.

While territorial expansion slowed, the U.S. emerged as a global power, reaching about 9.37 million km² with Alaska (1867) and Hawaii (1898). Overseas territories like the Philippines, Puerto Rico and Guam marked its transition from continental to international influence.