Was the Emancipation Proclamation Unconstitutional?

  1. The slaves were, legally, the property of the slave owners.

  2. The United States was at war with the Confederacy.

  3. A nation may, in the course of prosecuting a war, deprive its enemies of property without compensating them for it.

Therefore, depriving the southern states of its slaves was (legally) no different than raiding a supply depot and making off with bundles of rations and ammunition.