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;"
Invasion: \In*va"sion\, n. [L. invasio: cf. F. invasion. See Invade.] [1913 Webster]
1. The act of invading; the act of encroaching upon the rights or possessions of another; encroachment; trespass.