Final answer:
The Louisiana Territory originally belonged to Spain before being ceded to France in 1800, and was then purchased by the U.S. in 1803 for $15 million, doubling the size of the nation.
Step-by-step explanation:
The country that originally claimed the Louisiana Territory before it was purchased by the United States was Spain. Spain had control over it until the territory was ceded to France in 1800, during the turmoil of the Napoleonic Wars. Under Spanish rule, the U.S. had been denied access to New Orleans, which troubled Thomas Jefferson and other U.S. leaders who saw the Mississippi River and New Orleans as critical to American commerce and westward expansion. The Louisiana Purchase Treaty, facilitated by the negotiations of Robert Livingston and James Monroe, was signed on April 30, 1803, and the purchase concluded later in 1803, resulting in the U.S. paying $15 million for the vast territory, effectively doubling its size. This was a strategic move to ensure control over the Mississippi River and to fulfill Jefferson's vision of an agrarian society. The purchase also raised questions of constitutional authority and later challenges involving the expansion of slavery into the new territories.