During the American Revolution, Virginian George Rogers Clark, led the Illinois campaign against the British. Illinois Country east of the Mississippi River along with what was then much of Ohio Country, became part of Illinois County, Virginia, claimed by right of conquest. The county was abolished in 1782. In 1784, Virginia ceded its claims. Part of the area was incorporated in the United States' Northwest Territory. The name lived on as Illinois Territory between 1809 and 1818, and as the State of Illinois after its admission to the union in 1818. The residual part of Illinois Country west of the Mississippi was acquired by the United States in the Louisiana Purchase in 1803.