Final answer:
The Union needed a southern port to resupply its ships and maintain the blockade during the Civil War, which was vital for restricting Confederate trade and supporting Union military operations.
Step-by-step explanation:
The Union needed a southern port to resupply its ships and maintain the blockade. During the American Civil War, control of southern ports was crucial for the Union's strategy. Ports allowed resupplying and refueling of the Union navy, which was essential for maintaining a blockade that aimed to restrict the Confederacy's ability to trade, particularly with British support hoping for their cotton. Such a blockade was instrumental to the success of Union efforts, as it prevented the Confederacy from selling cotton abroad and importing necessary military supplies. The Union's capacity to hold these ports was also vital for logistical support of their military operations in the Confederate territory.