The Louisiana Purchase was justified constitutionally under the "Property Clause" of the United States Constitution. This clause, found in Article IV, Section 3, gives Congress the power to make rules and regulations regarding the property of the United States. As Louisiana was considered a territory or property of the United States, President Thomas Jefferson and his administration argued that the federal government had the authority to acquire and govern this land. Additionally, the Treaty Clause, found in Article II, Section 2, allows the President to make treaties, with the advice and consent of the Senate. The Louisiana Purchase was facilitated through a treaty with France, further supporting its constitutional justification.
