[Travis McGee]

U.S. Constitution, Article 4, Section 4:

"The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against invasion;"

[khelus]

You might be interested in Scalias dissention:
http://s3.documentcloud.org/documents/372493/scalia-statement.pdf

and interesting analysis of the decision:
http://www.krisannehall.com/index.php/blog/124-arizona-immigration-rulingdirect-assault-on-state-sovereignty

““ The most important point is that, as we have discussed, Arizona is entitled to have “its own immigration policy”—including a more rigorous enforcement policy—so long as that does not conflict with federal law. Today’s opinion, approving virtually all of the Ninth Circuit’s injunction against enforcement of the four challenged provisions of Arizona’s law, deprives States of what most would consider the defining characteristic of sovereignty: the power to exclude from the sovereign’s territory people who have no right to be there. Neither the Constitution itself nor even any law passed by Congress supports this result. I dissent.”” Scalia