<P> The Louisiana Purchase in 1803 , was negotiated with Napoleon during the presidency of Thomas Jefferson ; the territory was acquired from France for $15 million ( equivalent to $245 million in present - day terms ) . A small portion of this land was ceded to Britain in 1818 in exchange for the Red River Basin . More of this land was ceded to Spain in 1819 with the Florida Purchase , but was later reacquired through Texas annexation and Mexican Cession . </P>
in the first half of the nineteenth century the united states gained the most territory through
Louisiana Purchase