In 1863, a time pirate crash-landed in Mississippi carrying a futuristic bioweapon. He was later exposed to the virus and spread it to numerous Confederate soldiers causing symptoms comparable to zombies. This interfered with general Ulysses S. Grant's efforts in Mississippi when zombified soldiers killed Henry Scott causing a time aberration. The Legends arrived in Mississippi to fix the aberration. They completed Scott's mission and dealt with all of the infected soldiers fixing the aberration.
In an alternate timeline, the outbreak of confederate zombies ensured that Henry Scott's mission failed and that the Union lost the Battles of Champion Hill and Vicksburg. Due to these events, the Union lost the Civil War and the United States split into two nations with Mississippi a part of the Confederacy.