B) shift the objective of the Union Army from simply preserving the Union to abolishing slavery as well.
The Emancipation Proclamation, issued by President Abraham Lincoln in 1863, shifted the focus of the Civil War for the Union from merely preserving the nation to also including the goal of abolishing slavery. This strategic move aimed to weaken the Confederacy, as it declared that all enslaved people in Confederate states would be freed, thereby making emancipation a key aspect of the Union's war effort.