What finalized the southern borders of the continental US?

The southern borders of the continental United States were largely finalized through a series of treaties and purchases, most notably the Adams-Onís Treaty of 1819, which defined the boundary between the U.S. and Spain. This treaty ceded Florida to the U.S. and established a boundary that extended to the Pacific Northwest. Additionally, the Gadsden Purchase in 1854 further defined the southern border by acquiring land from Mexico, solidifying the current borders in the southwest. These agreements played a crucial role in shaping the geographical boundaries of the U.S. as we know them today.