The Louisiana Purchase of 1803 was the greatest accomplishment of Thomas Jefferson's presidency. His reservations about the deal stemmed from constitutional questions.
The Louisiana Purchase came about by accident. The United States merely wanted to secure control of New Orleans and small areas adjacent to it. Jefferson's representatives, upon arriving in France, were surprised to learn that Napoleon wanted to sell the entire Louisiana territory. The sale was a bargain for the United States, and it doubled the size of the country.
Jefferson did not know if the deal was constitutional, however. The Constitution did not have any instructions on the purchase of territory. Jefferson thought about a constitutional amendment to authorize the purchase, but the passage of an amendment would have taken too much time, and Napoleon might have changed his mind. Jefferson decided that the power to make treaties was sufficient.
The Louisiana Purchase, constitutional issues aside, was one of the most important events in American history.