Jefferson disliked the idea of purchasing Louisiana from France, as that could imply that France had a right to be in Louisiana. Jefferson had concerns that a U.S. president did not have the constitutional authority to make such a deal. He also thought that to do so would erode states' rights by increasing federal executive power.
On the other hand, he was aware of the potential threat that France could be in that region and was prepared to go to war to prevent a strong French presence there.
Throughout this time, Jefferson had up-to-date intelligence on Napoleon's military activities and intentions in North America.
Desperate to avoid possible war with France, Jefferson sent James Monroe to Paris in 1803 to negotiate a settlement, with instructions to go to London to negotiate an alliance if the talks in Paris failed.
Spain procrastinated until late 1802 in executing the treaty to transfer Louisiana to France, which allowed American hostility to build.
Also, Spain's refusal to cede Florida to France meant that Louisiana would be indefensible.
But in early 1803, continuing war between France and the UK seemed unavoidable. On March 11, 1803, Napoleon began preparing to invade the UK.
As Napoleon had failed to re-enslave the emancipated population of Haiti, he abandoned his plans to rebuild France's New World empire. Without sufficient revenues from sugar colonies in the Caribbean, Louisiana had little value to him.
Spain had not yet completed the transfer of Louisiana to France, and war between France and the UK was imminent.
Out of anger towards Spain and the unique opportunity to sell something that was useless and not truly his yet, Napoleon decided to sell the entire territory.
Although the foreign minister Talleyrand opposed the plan, on April 10, 1803, Napoleon told the Treasury Minister François de Barbé-Marbois that he was considering selling the entire Louisiana Territory to the United States. On April 11, 1803, just days before Monroe's arrival, Barbé-Marbois offered all of Louisiana for $15 million,equivalent to about $300 million in 2016 dollars, which averages to less than three cents per acre.
The American representatives were prepared to pay up to $10 million for New Orleans and its environs, but were dumbfounded when the vastly larger territory was offered for $15 million.
The Americans thought that Napoleon might withdraw the offer at any time, preventing the United States from acquiring New Orleans, so they agreed and signed the Louisiana Purchase Treaty on April 30, 1803.
On July 4, 1803, the treaty reached Washington, D.C..
The Louisiana Territory was vast, stretching from the Gulf of Mexico in the south to Rupert's Land in the north, and from the Mississippi River in the east to the Rocky Mountains in the west. Acquiring the territory would double the size of the United States, at a sum of less than 3 cents per acre.