The US doubled its territory in one deal because Napoleon needed cash for war

In 1803, the United States bought roughly 828,000 square miles of land from France for about $15 million — under three cents an acre — instantly doubling the country's size. Napoleon sold it not out of generosity but necessity: he'd lost his army to yellow fever and rebellion trying to retake Haiti, and needed funds to resume war with Britain. Thomas Jefferson, despite doubting he had constitutional authority to buy foreign territory at all, went ahead anyway.

— Peter S. Onuf, Jefferson's Empire: The Language of American Nationhood — University of Virginia Press, 2000
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