The Battle of Nelson was a military conflict in 2281.
Nelson was occupied by the NCR in an effort to secure a vulnerable stretch of land along the Colorado River, considered by General Lee Oliver to be a significant security gap between Camp Forlorn Hope and Camp Searchlight. The intermediary base was barely set up when Caesar's Legion invaded the town.
Two contubernia under the command of Decanus Dead Sea attacked Nelson, surprising the under-equipped soldiers stationed there. They swiftly overran the defenses and drove the NCR troopers out. Three soldiers were captured alive and promptly crucified.
By the time the Courier arrives in the Mojave Wasteland, the Legion has a firm hold on the town. Each side periodically sends out a party of skirmishers to try and weaken the other, but neither has any success in capturing territory and the battle lines are static. Because of the battle, morale at Camp Forlorn Hope has reached an all-time low. However, morale with the Legionaries at Nelson is high. At Caesar's command, Dead Sea's legionaries are on the defensive, the psychological effect of their presence being seen as too valuable to be risked in a concerted attack on Forlorn Hope.
Between Camp Forlorn Hope and Nelson is a small battlefield littered with dying NCR troopers and frag mines. Attempting to talk to them will only have them ask the Courier to perform a mercy killing. However, killing any one of them openly will turn both the other dying troopers in the field and the active NCR troopers within Camp Forlorn Hope hostile.
The Courier may choose to help improve the situation of the NCR at Forlorn Hope, ultimately allowing Major Polatli to order Sergeant Cooper to mount an offensive and retake the town in the quest Restoring Hope. The size of the force depends on the Courier's actions. Alternatively, the Courier may choose to help Caesar's Legion in the quest We Are Legion, assassinating the officers at Forlorn Hope to strike a far more serious blow to the NCR's morale and organization than Dead Sea's men could manage on their own. When the Second Battle of Hoover Dam starts, if the Courier helped the NCR, then Nelson will repel another Legion attack. If the Courier did nothing or completed the quest "We Are Legion," then it will fall under Legion control again.
The Battle of Nelson is only mentioned in Fallout: New Vegas.