Gettysburg: What was in the letter?

It hardly matters, since even if such a thing happened, Washington DC was crammed with troops and surrounded by forts, Grant had just taken Vicksburg and the entire army there and Rosecrans was driving Bragg into Georgia. So there would have been plenty of good news coming in, and no need to comply with such a letter.

And the chance of a 'destroying the enemy army' battle in the US Civil War was effectively zero. Nobody achieved one, aside from Grant at Appomattox, and he didn't fight a battle there. Lee never came close to doing that with the Army of the Potomac, and there is nothing at Gettysburg that would help. If he won, the Union Army would fall back and regroup closer to Washington, and get some reinforcements, and Lee would eventually have to retire to Virgina.

The Confederacy was fighting to become an independent nation. Although we call it the "Civil War," the term may cause confusion, sometimes. The Confederacy started a campaign to attack the North to force it to surrender, but it was not attempting to conquer and take over the North. It wanted the North to surrender and allow it to accept it's secession. The answer is resoundingly not the last. Washington DC would not be transferred.

The answer to the question should be the first. The Confederacy would have demanded a split, along the areas already defined by the state governments. The reason to do this is to end the war. If you split the territories up and give Union states to the Confederacy, people will keep fighting. It seems unimaginable the Confederacy would have demanded whole Union states, except perhaps Maryland, which was in some ways really not a Union state. It was conquered by the Union and proclaimed a Union state with only marginal popular support for the Union. It would be a good candidate to be split, as well.

This letter does seem quite difficult to find, but here is Lee's letter to the people of Maryland. I will post in full, since I think it answers what we can assume would be the contents of Lee's terms for surrender and its an interesting historical document:

To the People of Maryland: Headquarters, Army N. Virginia Fredericktown, 8th September, 1862

It is right that you should know the purpose that brought the Army under my command within the limits of your State, so far as that purpose concerns yourselves.

The People of the Confederate States have long watched with the deepest sympathy the wrongs and outrages that have been inflicted upon the citizens of a Commonwealth, allied to the States of the South by the strongest social, political and commercial ties.

They have seen with profound indignation their sister State deprived of every right, and reduced to the condition of a conquered Province.

Under the pretense of supporting the Constitution, but in violation of its most valuable provisions, your citizens have been arrested and imprisoned upon no charge, and contrary to all forms of law; the faithful and manly protest against this outrage made by the venerable and illustrious Marylanders to whom in better days, no citizens appealed for right vain, was treated with scorn and contempt; the government of your chief city has been usurped by armed strangers; your legislature has been dissolved by the unlawful arrest of its members; freedom of the press and of speech, of the Federal Executive, and citizens ordered to be tried by a military commission for what they may dare to speak.

Believing that the People of Maryland possessed a spirit too lofty to submit to such a government, the people of the south have long wished to aid you in throwing off this foreign yoke, to enable you to again enjoy the inalienable rights of free men, and restore independence and sovereignty to your State.

In obedience to this wish, our Army has come among you, and is prepared to assist you with the power of its arms in regaining the rights of which you have been despoiled.

This, Citizens of Maryland, is our mission, so far as you are concerned.

No constraint upon your free will is intended, no intimidation is allowed.

Within the limits of this Army, at least, Marylanders shall once more enjoy their ancient freedom of thought and speech.

We know no enemies among you, and will protect all of every opinion.

It is for you to decide your destiny, freely and without constraint.

This army will respect your choice whatever it may be, and while the Southern people will rejoice to welcome you to your natural position among them, they will only welcome you when you come of your own free will.

R. E. Lee, General Commanding.