Final answer:
The Civil War seriously undermined the issue of states' rights, shifting the power balance towards the national government.
Step-by-step explanation:
The single historical event that seriously undermined the issue of states' rights was the Civil War.
Before the Civil War, the Supreme Court ruling in Dred Scott v. Sandford stated that the national government lacked the authority to ban slavery in the territories.
The election of President Abraham Lincoln in 1860 led to secession of southern states and the conflict shifted from preserving the Union to ending slavery with the Emancipation Proclamation. The defeat of the South in the Civil War shifted the balance of power toward the national government, putting an end to the right of states to secede and imposing conditions for readmitting former Confederate states into the Union.
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