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When the Dred Scott case made it to the Supreme Court in 1856 (yep, it took 10 years), Taney sensed an opportunity to resolve the slavery question once and for all. By that time, the majority of Northerners wanted to see the institution limited to the states where it already existed, while the majority of Southerners wanted to see it spread, ideally as far and as wide as possible.
Taney, writing for the 7-2 majority, made two findings. The first was that the suit was fundamentally invalid, because Black men (and women) had no right to sue in an American court. The Chief Justice could have stopped there, but instead went on to also find that if the suit HAD been valid, Scott would still have lost, because the laws Congress had passed limiting slavery were invalid. In short, Taney said slavery was a state issue, and not a federal issue, and states and territories could do whatever they wanted when it came to legalization (sound vaguely familiar?). Problem solved!