This article is from the U.S. Civil War FAQ, by Justin M. Sanders firstname.lastname@example.org with numerous contributions by others.
Although the states remained part of the U.S., they had no loyal
governments, and the authority for the federal government to provide
mechanisms to erect loyal state governments was derived from Article IV,
Sec. 4 of the Constitution. That section provides that the United States
shall guarantee to each state a republican form of government.
Another important provision of the Constitution was Article I, Sec. 5
which provides that each House of Congress shall be the judge of the
qualifications of its members. This allowed the Congress to refuse to
seat delegations from former rebel states until the states had met the
conditions of the Reconstruction Acts.
The authoritative constitutional justification for reconstruction can
be found in the Supreme Court's decision in Texas v. White (74 U.S. 700)
delivered 12 Apr 1869. The entire decision is available on the Web at