The 11th Amendment to the Constitution reads:
"The Judicial Power of the United States shall not be construed to extend to any suit in law or equity, commenced or prosecuted against one of the United States by Citizens of another state or Subjects of any Foreign State."
In effect, the Dept. of Justice is sueing on behalf of illegal immigrants [Subjects of a Foreign State] against the State of Arizona.
The Dept. of Justice and the President who directed it are in clear and direct violation of the 11th Amendment.
Good article on the subject with video...