The American Civil War was a war that took place in the mid-19th century in the United States (Earth One). The conflict was primarily fought between the northern Union and the southern Confederate States of America, and was the bloodiest conflict to ever take place on American soil. According to historian Nate Heywood, roughly 3,000,000 soldiers fought in the war.
TX-90 and the night of the living dead
In 1863, at the height of the Civil War, a time pirate infected with the TX-90 virus crash-landed in Mississippi. The effects of the virus turned the pirate into a zombie-like creature, and he in turn spread the virus to a group of Confederate soldiers who found his crashed vessel. When the Legends arrived to answer the pirate's distress beacon, they encountered Union army dispatch Henry Scott, who was being chased by a group of the infected. Though the Legends easily defeated the zombies, Scott was mortally wounded in the process, and with his dying breath he tasked the team with his mission of stealing secret Confederate battle plans from the Collins Plantation. Mick Rory was also wounded in the battle, having been bitten by one of the infected soldiers, and he was taken aboard the Waverider for medical treatment.
While Ray Palmer and Martin Stein worked to create a vaccine to combat the effects of the virus, Jefferson Jackson volunteered to go to the Collins Plantation in Scott's stead, in order to prevent the aberration in the timeline that would occur should the Confederate battle plans not be delivered to General Ulysses S. Grant. Sara Lance determined that the mission was too dangerous for him to attempt alone, and thus sent Amaya Jiwe with Jackson as a precaution. Lance, meanwhile, sought to warn General Grant of the impending zombie threat, and along with Nate Heywood she traveled to the Union army camp in order to prevent further damage to the timeline. Though Grant was initially doubtful of their claims of undead soldiers, he quickly changed his mind when Lance deposited the head of a zombie as proof on a table in his tent. With an entire army of the infected within half a mile of the Union camp, Grant ordered his soldiers to guard the perimeter while he, Lance, and Heywood attempted to come up with a plan.
Jackson and Jiwe managed to reach the plantation in time to see Daniel Collins whip one of his slaves for being disobedient. Jiwe wanted to intervene, but Jackson insisted that they remain hidden behind a nearby tree in case rescuing the slave caused further damage to the timeline. Under the cover of nightfall, Jackson proceeded to disguise himself as a slave in order to infiltrate the plantation house during a party without arousing suspicion. He first searched through Collins' private study, but came up empty-handed. As he was about to search the rest of the house, however, Jackson bumped into one of the party guests in front of Collins himself. Thinking that Jackson was an insubordinate slave, Collins had him chained up in a barn outside, alongside other slaves whom he had deemed disobedient. Having lost contact with Jackson over their communication devices, Jiwe entered the house, and was almost immediately confronted by one of Collins' men. Remarking on how beautiful she was, he led her out to the barn where Jackson was being held so that she could be tied up for Collins to have his way with later that night. As soon as she saw Jackson, Jiwe activated her powers and knocked out Collins' man. After Jackson was freed, he urged Jiwe to free the others imprisoned in the barn as well; though Jiwe initially protested, as she felt doing so would damage the timeline as Jackson himself had warned before, she eventually consented when he proclaimed that the enslavement of African Americans was the 'real aberration'.
Together, they freed the slaves, and Jiwe led the majority of them to safety while Jackson returned to the plantation house with one of the slaves who claimed to know where Collins had hidden the Confederate plans. The zombie soldiers, meanwhile, had reached the plantation, and had proceeded to attack the guests inside the house. Jackson and the slave encountered Collins by a staircase, where he was attempting to fight off the horde with his firearms. Jackson offered to help if Collins would give him a weapon, but Collins refused, saying that he would never arm one of his slaves. He was subsequently killed by the zombies and eaten alive while Jackson and the slave escaped to safety upstairs with the battle plans. Jackson proceeded to drop a lit oil lamp on the swarm of the undead below, causing the house to erupt in flames; with the zombies burned to a crisp, Jackson and his companion returned outside to meet up wit Jiwe and the others, and together they took a moment of satisfaction in the destruction of the plantation house as a symbol of slavery.
At the Union camp, General Grant's men were quickly running out of ammunition against the onslaught of the undead. After seeing a crate containing nitroglycerin in Grant's tent, Lance devised a plan to use the compound to create an explosion. While the Union soldiers continued to draw the attention of the zombies, Lance was able to get one of the nitroglycerin crates into the middle of an open field. Heywood struck a flare and drew the attention of the zombies, leading them to the crate. When they began to attack him, he activated his powers and used the flare to ignite the nitroglycerin, creating a blast that destroyed all of the attacking zombies. His powers saved him from the explosion, and he was found alive and well underneath the pile of bodies, albeit covered in blood and guts from the infected soldiers.
Still aboard the Waverider, Palmer and Stein managed to create an effective aerosol antidote to the TX-90, which they used to cure a Rory. The Legends proceeded to spread throughout the infected areas of Mississippi, undoing the damage done by the virus. Jackson delivered the Confederate battle plans to General Grant, and introduced himself as Henry Scott, preserving the legacy of the Union hero in the timeline.
In an alternate timeline that spawned from the aberration caused by the death of Henry Scott, General Ulysses S. Grant never received the Confederate battle plans and the Union lost the war. As a result, the United States (Earth One) was split into two separate countries, divided between north and south.