The louisiana purchase was controversial however, it was the foundation for the westward expansion of the united states. the purchase doubled the size of the country and

The Louisiana Purchase was controversial because many felt that Johnson had overstepped his constitutional authority by buying land from another country. The purchase doubled the size of the country and ensured that if France did actually come to power at New Orleans, Americans in the west would be accorded the same commercial rights and privileges as under the Spanish. 

The Mississippi River was very important to the south because the transported almost all their cotton and other products on steamboats through it. The constitution didn't say anything about purchasing land from other countries so many doubted Jackson was allowed to.