Question

What Supreme Court decision in effect meant that the Constitution protected slavery?

Answer 1

Emancipation Proclamation

Answer 2

None. What you may be getting at is Dred Scott v. Sandford, decided in 1857, in which the Supreme Court held that slaves and descendants of slaves were not citizens of the United States, and therefore not subject to the Constitution at all. That meant that slaves and their descendants had no "standing" (right) to sue in Federal Court, and that Congress had no power to make law that affected them. All of their rights would be determined by the various state legislatures acting as they saw fit.

The infamous Dred Scott decision was vacated by the 14th Amendment, which explicitly declared that every person born in the United States (or naturalized) was a citizen of the United States.

There's a subtle point in that before Dred Scott it was unclear whether a person could be a citizen of a state but not a citizen of the United States, i.e. could he be subject to state law but not Federal law? Dred Scott said yes. The 14th Amendment said no. Dred Scott is widely considered a pro-slavery decision because it ruled that any interference in slavery by the Federal government -- either by Congress or the Federal Courts -- would be unconstitutional. (Presumably the President could still interfere to some extent by virtue of his power to handle the foreign affairs of the United States, and in fact this is the basis for Abraham Lincoln's Emancipation Proclamation, which declare slavery abolished in those regions of the country then in rebellion. It is to be noted that the Em. Proc. did NOT free slaves in areas NOT in rebellion, e.g. Maryland. It took the 14th Amendment to do that.)

The struggle over the Federal power to interfere with slavery was relevant because the great debate of the 1850s was the degree to which Congress could, or could not, prohibit the introduction of slavery into newly-formed territories and states out West. This itself was a proxy for the struggle between slave and non-slave states for control of the Federal government, since any bloc that controls a majority of the states ipso facto controls the Federal government. The slave-owning states feared (with good reason, as it turned out) that if non-slave states achieved a dominant position in the Federal government, they would act to destroy slavery in the slave states. Hence they were dead-set on preserving at least a strong minority in Congress by making sure enough new states admitted were slave states. (They cared much less about whether slavery was actually present in these states than that they would be part of the "states rights" bloc that would prevent Federal interference in slavery where it already existed.)

Dred Scott served to harden positions considerably on the eve of the Civil War, by convincing Northerners that the South was intransigent on the question of slavery in the territories, and convincing Southerners that their need to compromise was less than in fact it was.