Final answer:
The Union gained full control of the Mississippi River in 1863 after General Ulysses S. Grant successfully laid siege to Vicksburg, Mississippi, which resulted in a pivotal victory and strategic advantage during the Civil War.
Step-by-step explanation:
Union forces gained full control of the Mississippi River in 1863 following the surrender of Port Hudson, Louisiana, and General Ulysses S. Grant's successful siege of Vicksburg, Mississippi. The siege of Vicksburg was a pivotal military action that split the Confederacy in two and was crucial for achieving Union objectives during the Civil War. Vicksburg's strategic location on a bluff overseeing a significant bend in the Mississippi River made it a critical target for Union forces under Major General Ulysses S. Grant. Despite initial failures, Grant laid a siege to Vicksburg in May 1863 and after enduring over a month of Union bombardment and shortages, the Confederate defenders surrendered on July 4, 1863, marking a significant victory for the Union cause.