Republican Form of Government.
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.
ARTICLE IV, SECTION 4
This segment is called the Guarantee Clause because by its words the central government gives explicit guarantees to the states. One of these include security from foreign invasion and sustained Congress's prior commitment under the Articles of Confederation.
This guarantee is a part of the comprehensive policy, reflected in both the Constitution and articles.