What major land acquisition occurred between 1800 and 1825 that transformed U.S. territory?

The Louisiana Purchase is the major land acquisition that occurred between 1800 and 1825, significantly transforming U.S. territory. In 1803, the United States purchased approximately 828,000 square miles of land from France for about $15 million. This acquisition doubled the size of the nation and extended U.S. territory westward beyond the Mississippi River, which facilitated westward expansion and settlement. It provided access to vast natural resources and was essential for future economic growth and development.

This event also raised important questions about governance and the spread of slavery into new territories, setting the stage for debates that would shape the country's political landscape for decades. To summarize, the Louisiana Purchase was pivotal in expanding the United States geographically and politically during this period.