Louisiana Purchase
In 1795, the Pinckney Treaty with Spain allowed the US to use the Mississippi River but in 1901 Spain gave all of Louisiana back to France and therefore Americans no longer had access to the Mississippi river. Thomas Jefferson, elected in 1800, wanted to help out farmers. Jefferson persuaded James Monroe to talk to France in order to buy the Mississippi River but Napoleon sold all of France. Jefferson approved and that is the 1803 purchase. This ultimately reduced the power that Spain had in US and led onto the Lewis and Clark expedition. Controversy was that Jefferson shouldn’t have had the power, but he did it anyway.
http://mrnussbaum.com/history1/images/la.png