Which 1803 acquisition doubled the size of the United States in the history celebrated by America250?

The Louisiana Purchase doubled the size of the United States in 1803.

The United States acquired the Louisiana territory from France through a treaty signed on April 30, 1803. The purchase cost $15 million, including assumed claims, and transferred a vast region west of the Mississippi River to the United States.

The territory covered about 828,000 square miles, stretching from the Mississippi River toward the Rocky Mountains and from the Gulf of Mexico northward. It eventually became all or part of 15 present-day states and the province of Louisiana.

The deal is associated with President Thomas Jefferson and the diplomats Robert Livingston and James Monroe. Its boundaries and meaning were debated at the time, but the acquisition dramatically expanded the republic and encouraged later exploration and settlement.