Final answer:
Union forces needed to capture the city of Vicksburg to control the Mississippi River, which was strategic for splitting the Confederacy and aiding the Union victory.
Step-by-step explanation:
To take control of the Mississippi River, Union forces under Ulysses S. Grant needed to capture the city of Vicksburg. Vicksburg's strategic location on a bluff overlooking a narrow, slow, and winding point of the Mississippi River meant that whoever commanded the city could control river traffic. The Union saw capturing Vicksburg as essential to splitting the Confederacy in two, which was a crucial step towards winning the war. Despite setbacks and casualties from previous attempts, in May, Grant decided to lay siege to Vicksburg. This siege warfare was a long and tedious process but was perceived as necessary since other tactics had failed. Ultimately, this strategy proved successful, leading to the Union victory at Vicksburg after a protracted siege.