[GreenLanternCorps]

Nothing in the Constitution gave the United States authority over slavery until passage of the Thirteenth Amendment.

Prior to that it was a state issue.

The Emancipation Proclamation was legal because the slaves in the Rebel Confederacy were contraband of war, aiding in the unlawful rebellion.

The Thirteenth Amendment gave the Congress the power to ban it nationwide.

[EEGator]

How about the Preamble?

[Always A Marine]

The 10th Amendment, because the Constitution was silent on the issue until subsequent Amendments were passed and adopted. - Real Cynic No More

Nothing in the Constitution gave the United States authority over slavery until passage of the Thirteenth Amendment. Prior to that it was a state issue. -GreenLanternCorps

You are both correct, since the Constitution reserves to the States all powers that are not specifically enumerated to the federal government which the States established. States also had the right to secede under the same principle, but Lincoln chose to settle that issue on the battlefield where the Constitution yielded to force.