Which 1803 transaction nearly doubled the size of the United States?

The Louisiana Purchase was the 1803 transaction that nearly doubled the size of the United States.

The United States acquired the Louisiana territory from France through a treaty signed on April 30, 1803. The agreement transferred about 828,000 square miles of land for $15 million, or roughly three cents per acre. The territory extended from the Mississippi River to the Rocky Mountains, although its boundaries were not fully clear at the time.

President Thomas Jefferson supported the purchase because control of New Orleans and the Mississippi River was important to western farmers and national commerce. The acquisition also raised constitutional questions because the Constitution did not explicitly describe a power to buy foreign territory. Jefferson accepted the opportunity and sought congressional approval.

The purchase did not instantly settle every territorial claim or relationship with Indigenous nations. It began a long period of exploration, migration, conflict, and federal expansion. The Lewis and Clark Expedition started in 1804 to explore the newly acquired region.