Cut off trade to the West-- during war with Britain, Spain returned the Louisiana Territory to France under the control of Napoleon.
The US had been pushing westward and relied on the Mississippi River and New Orleans port for trade with the west. The Spanish allowed the US access to the river and port but under direction of the French, after the exchange, the treaty was ended. The US was concerned how this would cut off western areas. Diplomats were sent to France to negotiate a treaty for access and ended with an agreement to purchase the territory.
