Final answer:
Jefferson was worried because he recognized the strategic importance of controlling the Mississippi River and New Orleans for the economic and territorial expansion of the United States.
Step-by-step explanation:
Jefferson was worried about French control of the Louisiana Territory because he recognized the strategic importance of controlling the Mississippi River and New Orleans for the economic and territorial expansion of the United States. The Mississippi River and its tributaries provided access to the interior of the continent, and any westward expansion would involve the use of the Mississippi. By gaining control of the Louisiana Territory, the United States would have unrestricted access to the Gulf of Mexico and secure the vital trade route.
Additionally, French control of the territory could have posed a threat to American expansion and security. If France decided to establish a strong presence in North America, it could potentially limit American access to the Mississippi River and block American westward expansion.