At the time of French reacquisition of Louisiana in 1800, the United States was concerned about the strategic implications such a move could have on their newly acquired territory from the previous year's Treaty of San Lorenzo. The fears were that if the French had control of the port cities of New Orleans (via its ownership of Louisiana), the US would be at their mercy for trade and military purposes.
The U.S. was worried that the French presence would enable them access to the strategic waterways that linked the Gulf Coast to the rest of the continent. The U.S. saw the French reacquisition of Louisiana in 1800 as an aggressive act that could threaten their security and put them at a strategic disadvantage.