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.
The United States guarantees to every state:
a republican form of government (This is not defined. Most people agree it means that the people and their elected representatives run the government.)
protection from invasion
protection from riots or other violence within the state.