Final answer:
The Civil War is the best example of a conflict over state's rights, sparked by the secession of southern states over the power balance and the issue of slavery. The Union victory established federal supremacy, ending the right to secede and challenging federal laws.
Step-by-step explanation:
The event that stands as the BEST example of a conflict over state's rights is the Civil War. This monumental conflict arose from a complex set of issues, but a central one was the differing views between the North and the South regarding the balance of power between the federal government and the states, particularly on the issue of slavery. The Civil War was the ultimate showdown between national and state authority. The election of Abraham Lincoln led to the secession of eleven southern states, prompted by fears that the new administration would challenge the institution of slavery. The conflict solidified the Union's stance and the victory of the North put an end to states' rights to secede and to directly challenge federal laws like they had before, especially with slavery at stake.
Notable extensions of federal power before the Civil War include the Supreme Court cases of McCulloch v. Maryland and Gibbons v. Ogden, which began to shift the power balance. Moreover, after the war, this shift continued with the imposition of conditions for readmitting former Confederate states into the Union such as ratification of the Fourteenth and Fifteenth Amendments.