Which treaty defined the southern boundary of the United States at the 31st parallel?

The Pinckney Treaty, also known as the Treaty of San Lorenzo, was signed in 1795 between the United States and Spain and is notable for establishing the southern boundary of the United States at the 31st parallel. This treaty was significant as it resolved territorial disputes between the two nations and granted the United States the right to navigate the Mississippi River, as well as the right of deposit at New Orleans. The establishment of the 31st parallel as the boundary helped to clarify U.S. territorial claims and fostered relations with Spain at a time when the U.S. was expanding westward.

The Jay Treaty primarily dealt with unresolved issues from the Revolutionary War, particularly concerning trade and British military presence in the Northwest Territory. The Paris Treaty, or Treaty of Paris (1783), ended the Revolutionary War and recognized U.S. independence, but it did not set the southern boundary at the 31st parallel. The Treaty of Ghent ended the War of 1812 without addressing the boundary issues related to Spain. Thus, the Pinckney Treaty is the correct choice, as it specifically defined that southern boundary.