"[T]he question of whethor a particular amendment has... been lawfully ratified... would matter mightily with amendments as the three passed in the wake of the Civil War—the Thirteenth (abolishing slavery), the Fourteenth (defining citizenship and guaranteeing certain basic human rights to all persons in their dealings even with their own states), and the Fifteenth (abolishing racial qualifications for voting)—whose ratification by the legislatures of the former Confederate states was not exactly voluntary. Their acquiescence was secured by force, having been made a condition for their reentry into the Union from which they had attempted to secede."