This is roughly the shape of the Louisiana that Jefferson thought he had bought. As large as the portion west of the Mississippi appears, it was the long dog-leg of land east of that line, reaching into the northeast quarter of today's state of Florida, that was of far greater concern to him, and to the many Americans who had already moved west of the Allegheny Mountains by the end of the 18th Century.
Jefferson found out about Spain's secret transfer of Louisiana back to France, and in a letter to his good friend, the jurist Robert Livingston, written April 18, 1802, he clearly defined the anxiety the news aroused in him.
There is on the globe one single spot, the possessor of which is our natural and habitual enemy. It is New Orleans, through which the produce of three-eighths of our territory must pass to market, and from its fertility it will ere long yield more than half of our whole produce, and contain more than half of our inhabitants. France, placing herself in that door, assumes to us the attitude of defiance.
The only recourse, he reluctantly confided to Livingston, would be to "marry ourselves to the British fleet and nation."
He may well have wanted Napoleon to get wind of his concerns, for he neglected to seal the letter, but judiciously offered the French leader a way out:
If France considers Louisiana, however, as indispensable for her views, she might perhaps be willing to look about for arrangements which might reconcile it to our interests. If anything could do this, it would be the ceding to us the island of New Orleans and the Floridas.
It is not known whether Napoleon "got the message" or not.
Meanwhile, Jefferson was not above a little secrecy himself.
For further reading:
The full text of Jefferson's letter to Livingston will be found in Richard Hofstadter, Great Issues in American History: From the Revolution to the Civil War, 1765-1865 (New York: Vintage, 1958), 220-222.