Vicksburg

General Grant planned to take over the Mississippi River and divide the Confederacy in two, with help from William Tecumseh Sherman, and cavalry commander Benjamin H. Grierson. At Chickasaw Bluffs outside of Vicksburg, Sherman's advance had failed late in December 1862, but Greirson over the next months managed to get to Baton Rouge, Louisiana, destroying the railroad and taking prisoners, so that by the beginning of May Grant's mission was partially accomplished even though Vicksburg had not been taken.
Meanwhile, the Union Army of the Potomac and the Confederate Army of Northern Virginia were locked in battle. Both the Northern Army under and the Confederate Army under Lee divided their forces, but a Confederate line under "Stonewall" Jackson hit Hooker's center on May 2, causing Hooker to retreat on May 5. Thus, the Union Army failed to reach Richmond.

Chancellorsville

The next major engagement occurred at Chancellorsville where Union losses outnumbered Confederate by approximately 5,000, but Lee lost "Stonewall" Jackson.
Grant had continued to try to take Vicksburg from January to March in four attempts to cut canals or change the course of the river. In April, he devised a plan that included ferrying his men and marching into Mississippi behind Vicksburg. The Confederate Army would have to face him and fight or be driven into Vicksburg and cut off and forced to surrender. From April 16 to April 30 the plan was carried out with Grant cutting his own supply lines and heading deep into the enemies territory. The Confederate general Joseph E. Johnston hurried westward to protect the city, but Grant was already between him and the city, which he attacked on May19. While attacks failed, the city fell under siege which lasted more than six weeks, until July 4, with the Confederates surrendering 31,000 prisoners of war. Thus, the River was secured for trade and the Confederate Army was divided.
It was at this time that General Lee devised the bold plan to attempt victory on Northern soil in Pennsylvania, where lush provisions in the Cumberland Valley would fortify his ragtag army.