VICKSBURG, Miss. — Even 150 years later, Vicksburg is still overshadowed by Gettysburg — so much so, that the Mississippi city is having its Civil War commemoration a few weeks early rather than compete with Pennsylvania for tourist dollars around July 4.

Union forces waged a long campaign to conquer Vicksburg and gain control of the lower Mississippi River. The effort culminated in a concentrated military attack that started May 18, 1863, and a siege that started eight days later. Confederate forces surrendered the city on July 4.

The Battle of Gettysburg was fought July 1-3, 1863, and it produced a shockingly high number of casualties — 51,000 dead, wounded or missing....