In the late Summer of 1862 the Army of Northern Virginia was riding the crest of a wave of victory that was seemingly about to engulf the Federal capital itself.
General Robert E. Lee and his army, although victorious, was somewhat worse for wear. They had been marching and fighting since May, from down on the Peninsula withstanding McClellan's attempts to capture Richmond, to the latest battle at Manassas and his men were tired, ill clad and hungry. It would take some time to get them back into shape. But Lee had no intention of taking time. In a letter to President Jefferson Davis
dated September 3rd , Lee expresses his desire to move north into Maryland, but:
The army is not properly for an invasion of an enemy's territory.It lacks much of the
material of war, is feeble in transportation, the animals being much reduced,
and the men are poorly provided with clothes, and in thousands of instances
are destitute of shoes. Still, we cannot afford to be idle, and though weaker than our
opponents in men and military equipments, must endeavor to harass if we
cannot destroy them.  (partial text)
He had no desire to attack the Federals in their works about Washington for the reasons he stated in the above passage. He planned to provision and rest his army while threatening Washington and Federal troops in the Shenandoah Valley but he was intent on Maryland if it proved to be a feasible venture, and if not, the close proximity of the Confederate army to Washington would keep that city in turmoil and McClellans army occupied. Of necessity he could not wait long and on September 4th the Army of Northern Virginia began its crossing of the Potomac at Leesburg, Virginia. The relief of the citizens of Virginia was underway and the liberation of the good people of Maryland was at hand.
To be sure transferring the seat of war from Virginia had advantages, some of them very much worth the venture with a worn army, and Lee judged the risk beneficial to the Southern cause. There was always the possibility of moving beyond Maryland and into Pennsylvania. The movement of Lee's army would bring the Federal army North of the Potomac, and men of Maryland would flock to the colors of the Confederacy. The Confederate leadership had reason to believe the latter case was a real prospect. To the minds of the Confederate leaders the arrest of that states legislators and other citizens of high standing, the rioting in Baltimore in April 1861, and having several Maryland regiments serving in the Confederate armies was proof enough that Marylanders were very sympathetic to them and would eagerly join themselves to Lee's army. They believed that the citizens of Maryland needed a chance to freely express their will.  (They did, but not in a way hoped for, or in the numbers believed to be available.) All very good reasons to go North. The prospects looked very bright for such a move that late summer of 1862 and, back of it all, another prospect began to grow in Lee's mind. This new campaign was at its core a campaign for peace. No other reason would induce a general like Lee to risk losing his worn army across the Potomac other than that shining prospect. So Lee moved North.
On September 8th Lee issued a proclamation, over his own name, to the people of Maryland. In it he gives the reasons why the Confederate army had entered their state.
This, citizens of Maryland, is our mission, so far as you are concerned. No constraint upon your free will is intended; no intimidation will be allowed within the limit 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.  (Partial text)
These words indicate that Lee acknowledged the right of Maryland's citizens to make their own choice in the matter. Up to this point the Confederacy had been waging a war of self defense, now they were in Union territory. The wording also is suited to the political situation as a whole, and to the sensibilities of European observers, who, it was hoped, would perceive the Confederate army as liberators rather than invaders. Although the proclamation was written by Lee it conveyed the sentiments of a proclamation written by Jefferson Davis on September 7th. That proclamation was forwarded to Lee but it did not find him until several days after Lee had issued his own.
The Davis document is interesting in itself, but in its seventh item (it was written in bullet points) Davis declares that the state of Maryland could, if it chose, conclude a separate peace with the Confederacy, whereas Lee's document seems to reveal his feeling that Maryland rightfully belonged in the Confederate States and would be welcomed as if they had been held prisoners of the Union. The Davis Proclamation appears to be written in a way that appears to have foreign readers in mind, once again listing causes of the war and justification for entering Maryland.
It follows in its entirety:
RICHMOND, VA., September 7 [?], 1862
SIR: It is deemed proper that you should,in accordance with established usage, announce, by proclamation to the people of Maryland, the motives and purpose of your presence among them at the head of an invading army, and you are instructed in such proclamation to make known-
1st. That the Confederate Government is waging this war solely for self-defense; that it has no design of conquest, or any other purpose than to secure peace and the abandonment by the United States of their pretensions to govern a people who have never been their subjects, and who prefer self-government to a union with them.
2d. That this Government, at the very moment of its inauguration sent commissioners to Washington to treat for a peaceful adjustment of all differences, but that these commissioners were not received, nor even allowed to communicate the object of their mission; and that, on a subsequent occasion, a communication from the President of the Confederacy to President Lincoln remained without answer, although a reply was promised by General Scott, into whose hands the communication was delivered.
3d. That among the pretexts urged for continuance of the war, is the assertion that the Confederate Government desires to deprive the United States of the free navigation of the Western rivers, although the truth is that the Confederate Congress,by public act, prior to the commencement of the war, enacted that "the peaceful navigation of the Mississippi River is hereby declared free to the citizens of any of the States upon its boundaries, or upon the borders of its navigable tributaries, " a declaration to which this Government has always been, and is still, ready to adhere.
4th. That now, at a juncture when our arms have been successful, we restrict ourselves to the same just and moderate demand that we made at the darkest period of our reverses, the simple demand that the people of the United States should cease to war upon us, and permit us to pursue our own path to happiness, while they in peace pursue theirs.
5th. That we are debarred from the renewal of formal proposals for peace by having no reason to expect that they would be received with the respect, mutually due by nations in their intercourse whether in peace or in war.
6th. That, under these circumstances, we are driven to protect our own country by transferring the seat of war to that of an enemy, who pursues us with a relentless and, apparently, aimless hostility; that our fields have been laid waste, our people killed,many homes made desolate, and that rapine and murder have ravaged our frontiers; that the sacred right of self-defense demands that, if such a war is to continue, its consequences shall fall on those who persist in their refusal to make peace.
7th. That the Confederate army, therefore, comes to occupy to territory of their enemies, and to make it the theater of hostilities; that with the people themselves rests the power to put an end to this invasion of their homes, for, if unable to prevail on the Government of the United States to conclude a general peace, their own State government in the exercise of its sovereignty, can secure immunity from the desolating effects of warfare on the soil of the State by a separate treaty of peace, which this Government will ever be ready to conclude on the most just and liberal basis.
8th. That the responsibility thus rests on the people of --- of continuing an unjust and oppressive warfare upon the Confederate States--a warfare which can never end in any other manner than that now proposed. With them is the option of preserving the blessings of peace by the simple abandonment of the design of subjugating a people over whom no right of dominion has ever been conferred, either by God or man.
JEFFERSON DAVIS. 
Note the [?] in the date. Did Davis write this on the 7th or at a later date? One can speculate that it might have been written after the 8th, the date Lee issued his proclamation and after he wrote a letter to Davis encouraging the government to treat for peace with the Lincoln government. Item 4 of this document contains its sole demand; the independence of the Confederacy. This point appears out of place in a document supposedly tailored for the liberation of one state.
When looked at next to a letter written by Lee to Davis on September 8th, one may also speculate that the Davis Proclamation may in fact have had a broader implication and that Davis had received this letter before he wrote his own document to be issued in Maryland:
MR. PRESIDENT: The present position of affairs, in my opinion, places it in the power of the Government of the Confederate States to propose with propriety to that of the United States the recognition of our independence. For more than a year both sections of the country have been devastated by hostilities which have brought sorrow and suffering upon thousands of homes, without advancing the objects which our enemies proposed to themselves in beginning the contest. Such a proposition, coming from us at this time, could in no way be regarded as suing for peace; but, being made when it is in our power to inflict injury upon our adversary, would show conclusively to the world that our sole object is the establishment of our independence and the attainment of an honorable peace. The rejection of this offer would prove to the country that the responsibility of the continuance of the war does not rest upon us, but that the party in power in the United States elect to prosecute it for purpose of their own. The proposal of peace would enable the people of the United States to determine at their coming elections whether they will support, those who favor a prolongation of the war, or those who wish to bring it to a termination, which can but be productive of good to both parties without affecting the honor of either.
R.E. Lee 
It is my belief that the Davis document was in fact written after the 7th of September, and after he had received Lee's letter and it was intended to be a document to use for securing terms of peace between the two countries. Note two glaring points in both documents that point to this. The first is from Lee's letter to Davis dated the 8th regarding the prospect of a peace settlement:
Such a proposition, coming from us at this time, could in no way be regarded as suing for peace; but, being made when it is in our power to inflict injury upon our adversary, would show conclusively to the world that our sole object is the establishment of our independence and the attainment of an honorable peace. (Lee letter)
Comparing it to this from the Davis document:
That now, at a juncture when our arms have been successful, we restrict ourselves to the same just and moderate demand that we made at the darkest period of our reverses, the simple demand that the people of the United States should cease to war upon us, and permit us to pursue our own path to happiness, while they in peace pursue theirs. (Point 4)
They are very similar in wording and intent, so much so that it seems Davis wrote his proclamation using it as a basis. It is worded broadly enough to encompass the end of hostilities and assumes that the Confederate armies had reached “a juncture when our arms have been successful” in Maryland and beyond what they had already achieved that summer.
Another point worth noting in similarity:
The rejection of this offer would prove to the country that the responsibility of the continuance of the war does not rest upon us, but that the party in power in the United States elect to prosecute it for purpose of their own. (Lee letter)
...if such a war is to continue, its consequences shall fall on those who persist in their refusal to make peace. (Davis document, point 6, last line)
Again they are strikingly similar.
In an effort to solidify my theory that the main goal perhaps the only goal of the Maryland Campaign was a final peace, I looked at several other letters between the two men during the same time frame.
In a letter to Davis dated September 9th , Lee acknowledges receipt of a letter from Davis, written on the 7th ,which states Davis' plan to go to Leesburg and meet with Lee. Lee states that he would be happy to see the president and consult with him “upon all subjects of interest but makes no mention of the Davis document. In fact Lee sends Major Walter Herron Taylor back to prevent Davis from coming up.
On the 12th, Lee writes another letter to the president apprising him of the affairs of the army, and only late in the letter does he tell Davis that he is forwarding to him a copy of his own address to the citizens of Maryland.
Finally, in a letter dated the 13th, Lee make mention of the Davis document, and says of his own. “I have not gone contrary to the views expressed by you on the subject.” 
So why, then, did it take so long for Lee to acknowledge the receipt of the Davis plan? The Army of Northern Virginia was rather busy at the time and Lee can be excused for not acknowledging the receipt of Davis' proposals sooner but there were other letters from Lee to Davis between the 8th and 13th (as far as the O.R.'s show) but none make mention of the presidents document. Given this, I believe Davis wrote his proclamation after receiving Lee's letter of the 8th, that he worded it broadly so as to encompass a peace proposal to the United States, and that the campaign itself was intended to place the Confederacy on the strongest footing possible to offer such a proposal. After looking at these documents I believe this to be the main reason Lee moved north. Hopes of political advantage to come from the Fall elections, destruction of McClellan's army, or even foreign recognition were only hopes, the other benefits although much needed and modestly gained were of secondary concern to Lee and Davis. Both men intended the Maryland Campaign to be the final campaign of the war, with peace and Southern independence gained. Lee and Davis were willing to risk all to achieve it and confident of the end result. Lee's letter shows that he wanted it to be sooner before more blood was shed, rather than later with political change and pressure. He was willing to accept either course, but as the events would develop, he would not gain the desired result. The dream died on Marylands bloody fields.
- War of the Rebellion, Records of the Union and Confederate Armies, Series1, Volume 19, part 2, pages 590-591 (Noted from here as O.R.)
- Ibid page 592. Letter of September 4th, 1862, Lee to Jefferson Davis
- R.E. Lee, Volume 2, Freeman, Douglas S., page 351
- O.R. Vol 19, Part 2, page 601-602
- Ibid. Page 598-599
- Ibid, Page 600
- Ibid, Pages 602, 605