Truth be told the order to invade Pennsylvania came from Jefferson Davis feeling the stress from the western frontiers Lees army had been whooping yankee ass and would have faired better doing so cutting off and defeating individual threats and consolidating as they did in the valley campaign
This is true that the final approval for Lee to invade the north did come directly from Jefferson Davis. However, this was not a rash decision as there was a lot of debate whether to have Lee with the majority of his army stay in the area of northern Virginia, protecting Richmond and send Longstreet with his corps as quickly as possible to join with Johnston in Mississippi against Grant to break the siege of Vicksburg. Davis agonized over this choice. Lee was for invading the north and putting and end to the war, which, given his army's track record of defeating the Federals, was a sound proposition. The recent loss of Jackson was a serious blow to the Confederate command structure. However, it was not something that could not be overcome. Other competent officers were available. Maybe not quite as skilled as Jackson was, but this is war - Jackson is dead - you have got to move on. Lee had an army that was capable and accustomed to being victorious. The Federal Army of the Potomac's morale and confidence at that time was generally low. I can plainly see the logic in taking the offensive.
However, several in Davis' cabinet suggested holding the Federals in check north of Richmond and relieving Vicksburg, saving most of Louisiana,Texas, Arkansas, the Confederate Arizona Territory, and Indian Nations from being effectively cut off from the rest of the Confederacy. Meanwhile, Gen. Pemberton was holding on as best he could to Vicksburg that Grant had under siege. Johnston was massing troops as best he could from all parts of the southeast that could be spared in an effort to break the siege, but it was a near impossible task as none could be had from west bank of the Mississippi River that was then controlled by Federal forces. Gen. Bragg was reluctant to send Johnston any support and moving troops from the defenses of Mobile was unwise because the Federals by that time occupied both New Orleans and Pensacola, on the west and east flanks of Mobile, and were constantly threatening attack from either, or both. Johnston knew that he was facing overwhelming numbers and had to plan his attack on Grant carefully as he would have only one chance at it. Additionally, according to the last message that was received from Pemberton to Johnston via courier, he reported he estimated that he could hold out until mid-July. This was a miscalculation on Pemberton's part. The results were that Johnston was unable to assemble a force large enough to engage Grant and relieve the siege before Pemberton's army ran out of food and were too weak to fight their way out. They had plenty of arms and ammunition, but food had been exhausted. Johnston did not know the extent of the food shortage and was making his plans by his last communication with Pemberton.
So, this was the general situation. It was believed, incorrectly, that Vicksburg could hold out until it could be relieved with troops from the southeast. If Lee was successful, and he and his army had the track record of success, in defeating the Federals on northern soil and taking Baltimore, Philadelphia, and/or Washington itself, they could demand an end to the war and independence. If Lee had taken Washington, it would have made no difference what happened at Vicksburg. Remember, the Confederacy never had any intention whatsoever of conquering or destroying the United States government or it's cities, etc. They simply wanted their independence to govern themselves. They wanted out of what they believed had become a bad marriage, if you will. We can play countless hours of "woulda, coulda, shoulda", but that didn't happen. Looking back with 20/20 hindsight, if Lee had stayed in Virginia with most of his army and sent Longstreet with his corps to Mississippi and they, with Johnston and Pemberton's forces, had defeated Grant, that in itself would not have ended the war. However, Grant was on the verge of being relieved of command due to his past failures to take Vicksburg. I doubt his career as an army commander would have continued. Would at that point a frustrated U.S. government agree to peace? Maybe. Frankly, I don't know.
People in the north by that time were very weary of the war. They had recently lost several large eastern battles with horrendous casualties and had repeatedly failed to take Richmond. Now their great western army has been defeated. So now, what do the Confederates do militarily? Three possibilities as I see it: 1) send Longstreet's Corps back to Virginia?, 2) attack and retake New Orleans?, or 3) move north towards St. Louis?. My choice would be New Orleans. With the defeat of the western Federal army and the retaking of New Orleans, in my opinion, this would bring about foreign recognition of the Confederacy by England and France and most probably a peace treaty with the U.S. and independence. However, like I earlier said, that ain't what happened. 20/20 hindsight. It could easily have happened.
JMHO,YMMV
