Final answer:
The battle that was not an offensive move by the Confederacy during the American Civil War is Vicksburg. This was a Union campaign to control the Mississippi River, unlike the other battles mentioned, which were Confederate offensives.
Step-by-step explanation:
The battle that was NOT an offensive move by the Confederacy during the American Civil War is Vicksburg. The Siege of Vicksburg was a Union campaign to gain control of the Mississippi River and split the Confederacy in two. It was strategically critical to the Union's Anaconda Plan, which aimed to surround the southern states and cut off resources and movement. The other battles listed, namely Gettysburg, Chancellorsville, and the Seven Days' Battle, were indeed offensive operations by the Confederacy. Notably, the Confederate defeat at Gettysburg marked a turning point, as it was the last attempt by General Lee to invade the North, while the Union's success at Vicksburg fortified their control of the Mississippi River.