From The Emancipation Proclamation of September 22, 1862:
That on the first day of January in the year of our Lord, one thousand eight hundred and sixty-three, all persons held as slaves within any State, or designated part of a State, the people whereof shall then be in rebellion against the United States shall be then, thenceforward, and forever free; and the executive government of the United States, including the military and naval authority thereof, will recognize and maintain the freedom of such persons, and will do no act or acts to repress such persons, or any of them, in any efforts they may make for their actual freedom.
General Robert E. Lee surrendered his Army of Northern Virginia on XX at Appomattox Courthouse on April 9, 1865.
And then:
After President Lincoln issued the Emancipation Proclamation, many enslaved people would remain in bondage for two and a half years. Even two months after the Confederacy surrendered at the Appomattox Courthouse, slavery persisted in Texas. That is, until Union forces arrived to enforce the Proclamation.
In Galveston on June 19, 1865, U.S. Army General Gordon Granger issued General Order Number Three statings: “The people of Texas are informed that, in accordance with a proclamation from the Executive in the United States, all slaves are free.” Later in December, the Thirteenth Amendment was ratified to abolish slavery throughout the United States.
You can read the original document here.
The US Army had to, in effect, invade Texas to enforce Lincoln's proclamation.
Then this - The Thirteenth Amendment:
Section 1. Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.
Section 2. Congress shall have power to enforce this article by appropriate legislation.
Ratified by the Congress on December 6, 1865.
