Excerpts from Books and Wikipedia
"Thomas Jefferson was the first U.S. president to be inaugurated, in 1801, in the country's new capital of Washington, D.C. The land purchase was his most famous single achievement. Ironically, for a man who had proclaimed his disapproval of a strong federal government, it involved in unprecedented exercise of presidential power for which the Constitution gave him no warrant." [1001 Days]
"Neither Britain nor Spain recognized the legality of the Louisiana Purchase, since France had violated the Treaty of San Ildefonso (1800) by selling Louisiana to the United States." [Howe: What Hath God Wrought, p. 16]