Why were the Civil War battles of Port Hudson and Cold Harbor so important?

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Although these two battles took place in different theatres of the war and nearly one year apart, they proved to be two of the bloodiest and one-sided battles of the American Civil War. The victory by the Union army during the Siege of Port Hudson (June-July 1863) cleared the Mississippi River of the Confederacy's final defense, following on the heels of Gen. Ulysses S. Grant's successful victory at Vicksburg just one week before. As for the Battle of Cold Harbor (June 1864), it proved to be one of Grant's greatest blunders, and the Confederate victory was Gen. Robert E. Lee's last great--and his most lopsided--victory.

PORT HUDSON.  Located south of Vicksburg on the Mississippi, the Louisiana fortress was one of the strongest on the river. Although lightly defended (by less than 8000 men), the Confederates repulsed several attacks by Union forces of nearly 40,000 men. The Union commander, Gen. Nathaniel Banks, eventually settled on a siege, and the Confederates only gave up the fight after learning of Gen. John Pemberton's surrender at Vicksburg in early July 1863. The Confederates who defended Port Hudson inflicted more than 5000 Union casualties while losing only about 500 men prior to the surrender of the 6500 man garrison. 

COLD HARBOR.  In later years, Grant admitted his mistake at the repeated attacks on Lee's heavily defended position at Cold Harbor, Virginia, the site of the previous Battle of Gaines Mill during the Seven Days Battles of 1862. The two main Union assaults of June 1 and June 3 were two of the bloodiest of the war, thanks in part to poor Union reconnaissance, muddled orders, and the strong Confederate position. Union commanders and soldiers both recognized the futility of the assaults that Grant ordered: Many soldiers pinned their name and hometown addresses inside their uniform jackets before the final assault so that their bodies could be identified and sent home for burial. In all, Grant's Army of the Potomac (which outnumbered Lee's Army of Northern Virginia by nearly 2-to-1) suffered nearly 13,000 casualties. Lee's forces lost only 83 killed. Although the battle cemented Grant's reputation as a "butcher" of troops and caused an outcry of anti-war sentiment in the North, Cold Harbor kept Lee trapped, and he was forced to send his army into the trenches at Petersburg in order to defend the Confederate capital of Richmond.