United States territorial acquisitions
The Panama Canal Zone was an unorganized US territory located within the Republic of Panama. It was established under the Hay–Bunau-Varilla Treaty in 1903 and disestablished in 1979 under the Torrijos–Carter Treaties. Panama gained full control over the Panama Canal in 1999.
Panama Canal Railway
Following the United States' acquisition of Alta California in 1846 and the Oregon Territory in 1848, following the Mexican–American War and with the prospective movement of many more settlers to and from the West Coast, the United States again turned its attention to securing a safe, reliable, and speedy link between the Atlantic and Pacific oceans. In 1846 the United States signed a treaty with Colombia (then the Republic of New Granada) by which the United States guaranteed Colombian sovereignty over Panama and was authorized to build a railroad or canal at the Panamanian isthmus, guaranteeing its open transit.