The Battle of Gettysburg, which became the largest battle ever fought in the U.S., started out as a chance encounter between the Union and Confederate Forces.After his victory at Chancellorsville in Virginia, Confederate commander Lee decided to focus on invading the North in what he called the Gettysburg Campaign.The plan was to try and get some leverage in the North by forcing Northern politicians to stop prosecuting the war.fter his victory at Chancellorsville in Virginia, Confederate commander Lee decided to focus on invading the North in what he called the Gettysburg Campaign. The plan was to try and get some leverage in the North by forcing Northern politicians to stop prosecuting the war.