The year is 1863. The Union and Confederate Armies are moving up through Maryland, and into Pennsylvania. Robert E. Lee hopes that a victory in the North will bring the British and French onto the side of the Confederacy to pressure the Union to end the Civil War. The Army of Northern Virginia began moving north on June 3rd, with the goal of a battle in the North. The Army of the Potomac followed suit several days later, on June 13th.
In our timeline, the Union and Confederate armies met, almost by chance at a small town known as Gettysburg. But in this timeline, Lee makes the decision to keep part of Jeb Stuarts cavalry close to the main body of the Confederate Army, as well as deciding to move on Harrisburg once given the position of the Union Army.
These two events lead to the Battle of Harrisburg, with the Army of the Potomac and the Department of the Susquehanna facing off against the Army of Northern Virginia outside of the city of Harrisburg. In the panic of the advancing Confederacy, Pennsylvania's state government evacuates to Reading, Pennsylvania.
The five day battle inflicted heavy casualties on both sides, and prevented the Confederacy from launching an attack on Philadelphia, or Washington D.C. as Jefferson Davis hoped, but the Union Army was unable to take advantage of the situation and destroy the Confederacy. They were successful in pushing them out of Pennsylvania, and back into Virginia. Out west, Grant successfully captured Vicksburg, but lost a large portion of his military forces, and was unable to hold the city for a long period of time. President Lincoln gave Grant order
With a general stalemate settling along the front of the war, political, business, and religious leaders called for a peace treaty and an end to the war. After long negotiations, and fears of a return to war, General Ulysses S. Grant and General Robert E. Lee signed the Treaty of Alexandria. Indemnities were relatively small, with the Confederates not wanting to create permanent enemies with the North. Robert E. Lee was particularly adamant about not punishing the North, preferring to treat them with respect, and eventually mend relations.