Lincoln's note to Maj. Gen. Henry Halleck was written July 7, 1863, after the defeat of Gen. Robert E. Lee at the Pennsylvania battle site and the fall of Vicksburg, Miss., ABC News reported Thursday.

Historians knew of the note because Halleck cited it in a telegram, said Trevor Plante, the Civil War specialist who found it three weeks ago.

"We have certain information that Vicksburg surrendered to (Gen.) Grant on the (Fourth) of July," the note said. "Now, if Gen. Meade can complete his work so gloriously prosecuted thus far, by the (literal) or substantial destruction of Lee's army, the rebellion will be over."

Meade took over Union forces just before Gettysburg, a battle cited as the turning point of the Civil War.

Plante found the document while gathering information for a Discovery Channel documentary about Gettysburg.

"I was looking for something else and frankly where I found it was in an obscure place," he said.