The numbers that stand out:
Louisiana Purchase (1803): 2.14 million km²
Texas Annexation (1845): ~1.01 million km²
Alaska Purchase (1867): ~1.53 million km²
Greenland: ~2.17 million km²
The US hasn’t added major territory in nearly a century.
The last significant US territorial acquisition came in 1917, when Washington purchased the US Virgin Islands from Denmark for $25 million. The islands — St. Thomas, St. Croix, and St. John — were acquired during World War I for their strategic value in protecting Caribbean sea lanes and the approaches to the Panama Canal. They remain unincorporated US territories.