The Louisiana Purchase expanded the United States in 1803.
The United States acquired the Louisiana Territory from France through a treaty signed on April 30, 1803. The agreement was completed during Thomas Jefferson’s presidency and transferred a vast region west of the Mississippi River to the United States.
The purchase cost $15 million, including the assumption of certain claims by American citizens against France. It roughly doubled the size of the United States and strengthened American control of the Mississippi River and the port of New Orleans.
The purchase did not instantly create all the modern states within the territory. Much of the land was later surveyed, organized into territories, and admitted as states over many decades. The acquisition also intensified conflicts involving Native nations and expanded the institution of slavery into new political debates.