Napoleon, the leader of France, offered to sell America the land of the Louisiana Territory. The port at New Orleans was an important ,even vital, port for American trading. After a bunch of arguement between Spain, France, and America, Napoleon just decided to sell the land of the Louisiana Territory to America. There are three reasons historians think he may have done this
*1 --- He realized the Americans really needed the Port of New Orleans
*2 --- A revolt in the West Indies lessened his enthusiasm for owning a colony in America
*3--- The war that the French had fought against British had taken an economic toll on the French.
President Jefferson bought the Louisiana Territory for 15 million dollars.
Hope this helps :)