The Battle of the Divide (also known as the Battle of the Two Couriers) was the conflict between the Courier and Ulysses in the year 2281. A long trail of discovery, philosophy, and history, it took place in the Divide in New California, and decided the very fate of the east and west for decades, and possibly centuries to come.
When the Courier had unsuspectingly destroyed the towns of Hopeville and Ashton with Enclave technology from Navarro, a Caesar's Legion agent named Ulysses survived the explosions after medical eyebots revived him due to his American flag duster. The entire event changed Ulysses, and brought him to many revolutionary realizations. This disaster nearly killed Ulysses, but he was saved by several medical eyebots, who had also been activated by the copy of ED-E. They recognized the flag of America on his back and saved him. Ulysses' life was changed that day, showing him how a single individual could change history, or erase it. As a survivor of the Divide, he held the Courier responsible for the destruction of the place that could have been his true home; of something "larger than the Bear, greater than the Bull." At the same time, it inspired him, showing just how great of an impact a single person can have on history. In 2277, he returned to Caesar, learning that the First Battle of Hoover Dam ended in the defeat of Caesar's Legion. There, he was assigned a new duty.
Ulysses was determined to take revenge and sought to reshape the post-apocalyptic world single handedly, much like the Courier reshaped the newly named Divide by unleashing nuclear fire upon it. At some point before the Second Battle of Hoover Dam, Ulysses contacted the Courier via a single message: the coordinates for the canyon wreckage west of Primm, and the words "Courier Six - Ulysses", wanting to destroy their new home in his reshaping process of America before their eyes, as the Courier did for him with the Divide. The Courier answered the call and confronted Ulysses at the Divide. There, under the Old World flag and the torn skies, the two decided the fate of America.
Once the Courier arrived at the Divide, they met a clone of a robot named ED-E, who was cloned by the Divide's computers when he passed through the area. The robot assisted the Courier in traversing the missile base. Ulysses then contacted the Courier through ED-E and discussed the situation. Afterwards, the Courier retrieved a laser detonator and the Red Glare. Using the laser detonator to destroy warheads blocking their path, the Courier continued on and passed the High Road, slaughtering packs of deathclaws and tunnelers. Along the way, the Courier accidentally launched a nuclear warhead that entirely destroyed an area of the Divide over again, creating the Courier's Mile.
Once the Courier had finally reached Ashton, Ulysses confiscated ED-E, and brought him back to his temple. The Courier had to reach the temple alone. Once the Courier entered the temple, they were reunited with ED-E, who was either freed from the bot pods, or left alone. The Courier (and ED-E, if freed) entered the missile silo where Ulysses awaited.
Ulysses revealed to the Courier that he had set the Divide's nuclear missiles to launch within minutes and destroy the entirety of the Long 15, cutting off the NCR in the Mojave Wasteland and dooming the Republic as well as the denizens of the Mojave. There was no way to deactivate the launch, except with ED-E. They could however be redirected elsewhere. The Courier and Ulysses then discussed the matter extensively. What became of this is unknown to history.