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.
How about the Preamble?
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.