<P> A political crisis in 1840s Rhode Island, the Dorr Rebellion, forced the Supreme Court to rule on the meaning of this clause . At the time, the Rhode Island constitution was the old royal charter established in the 17th century . By the 1840s, only 40% of the state's free white males were enfranchised . An attempt to hold a popular convention to write a new constitution was declared insurrection by the charter government, and the convention leaders were arrested . One of them brought suit in federal court, arguing that Rhode Island's government was not "republican" in character, and that his arrest (along with all of the government's other acts) was invalid . In Luther v. Borden, the Court held that the determination of whether a state government is a legitimate republican form as guaranteed by the Constitution is a political question to be resolved by the Congress . In effect, the court held the clause to be non-justiciable . </P> <P> The Luther v. Borden ruling left the responsibility to establish guidelines for the republican nature of state governments in the hands of the Congress . This power became an important part of Reconstruction after the American Civil War . The Radical Republican majority used this clause as the basis for taking control of the ex-Confederate states and for promoting civil rights for freedmen, plus the limiting of political and voting rights for ex-Confederates, abolishing the ex-Confederate state governments, setting guidelines for the readmission of the rebellious states into the Union . </P> <P> While the Supreme Court's holding in Luther v. Borden still holds today, the Court, by looking to the Equal Protection Clause of the Fourteenth Amendment (adopted 19 years after Luther v. Borden was decided), has developed new criteria for determining which questions are political in nature and which are justiciable . </P> <P> (...) and (the United States) shall protect each of them (the States) against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence . </P>

What are the states obligations to each other