Why was the United States able to acquire the Louisiana territory in 1803?

The Louisiana Purchase was the acquisition of the territory of Louisiana by the United States from France in 1803. In return for fifteen million dollars, or approximately eighteen dollars per square mile, the United States nominally acquired a total of 828,000 sq mi. However, France only controlled a small fraction of this area, with most of it inhabited by Native Americans; for the majority of the area, what the United States bought was the "preemptive" right to obtain Native American lands by treaty or by conquest, to the exclusion of other colonial powers. The total cost of all subsequent treaties and financial settlements over the land has been estimated to be around 2.6 billion dollars.

i think that the right answer is the aztecs