The Federal Department of the Interior’s Bureau of Land Management put up this sign somewhere south of Phoenix, Arizona, United States of America, to cover their ass because the Federal Department of Homeland Security’s largest and most complex component ain’t gettin’ ‘er done.
The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion;
See Signs of Border Trouble in Arizona and Nogales or Somalia?
UPDATE 06162010: Uptick in Violence Forces Closing of Parkland Along Mexico Border to Americans
Civilian Irregular Information Defense Group 
All of 4.4 is relevant (and makes the case even stronger):
“The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.”
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