Massive purchase of land from France.
The Louisiana territory was sold by France to the US for $15 million, purchased under Thomas Jefferson and added roughly 800,000 square miles to the US. Sold because Napoleon did not want to manage it anymore and was more interested in his own empire. Doubled the size of US territory.
Important because Thomas Jefferson's move was considered unconstitutional since he had bend the rules and make it so he could buy the land because it wasn't in the constitution, seen as hippocratic.
Anything else--supplemental info, memory pegs, etc.