The Battle of the Tollen Dam took place around fourteen years after Emergence Day. The Locust Horde organized a massive offensive to take the Tollen Dam, the center of the COG's electric infrastructure in the dwindling COG territory. The Coalition would reinforce the dam from several nearby COG outposts including Fort Reval. The Locust Horde was able to besiege the dam, trapping the untold numbers of Gears inside. This led to a gradual loss in supplies and food, eventually causing starvation. Despite sending the famed Delta-One led by Marcus Fenix, COG High Command was unable to break the siege. The Dam was overrun and taken offline by the Horde, causing the COG to abandon several cities and outposts due to the power loss. The COG would later downplay the losses suffered during the battle, saying that only a few dozen Gears are trapped inside.
After the sinking of Tollen, a Locust attack on the Tollen Dam became a real possibility. Soon after, the Locust forces started amassing near the dam. Knowing that the attack would have dire strategic consequences as the dam powered many cities in the region including the industrial town of Speyer, the COG High Command redeployed all Gears stationed in Fort Reval, Speyer and Tollen to the dam with orders to hold it at any cost. Because the reinforcements and supplies could not be guaranteed, the soldiers were told to take as much food and ammunition as they could carry with them. While preparing for the attack, the Gears built many sandbag walls and makeshift barricades, turning the Tollen Dam into a fortress.
- "Ran out of food last week. So hungry I can't even lift my Lancer."
- —Sgt. Boyd Packer, in his final letter.
Shortly after the COG forces were redeployed to the dam, the Locust completely surrounded and besieged the power plant. Instead of assaulting the dam, the Locust decided to starve the defenders out. Hunger became a huge issue among the defenders.
- "[...] a few dozen Gears were trapped inside."
"So, how did you get them out?"
- —Marcus Fenix and JD Fenix discussing the battle.
After a few weeks of siege, the COG High Command sent a relief force to the dam. Despite heavy fighting, the reinforcements consisting of Delta-One and at least one King Raven helicopter were not able to break the siege and had to abandon the mission. The situation inside the dam was desperate and pushed at least one Gear - Sergeant Boyd Packer to commit suicide.
When the relief attempt turned out to be a failure and the defenders became weak due to exhaustion and hunger, the Locust stormed the dam. Heavy fighting followed with the Locust overrunning the site and killing all of its defenders.
The battle of the Tollen Dam ended with a decisive strategic Locust victory. The loss of the dam caused a regional power outage, forcing the COG to evacuate Speyer which dealt a heavy blow to it's industry as the town housed the last remaining COG factories such as Speyer Metalworks. The city's personel was redeployed to Ilima for reassignments.
The human casualities are difficult to determine. The COG officially stated that the dam's garrison consisted of "a few dozen Gears", but the number could actually be much higher. The Locust forces suffered from moderate casualties.
Twenty eight years after the battle, the Tollen Dam became a battlefield once again. Marcus Fenix - one of the battle's few survivors led a reconnaissance mission to the plant and a team of Outsiders clashed with the Swarm. During the mission, the team restored the power to the dam by using the single remaining intact turbine and the wind that restored the power to Fort Reval, Speyer, Ilima, Tollen, Bronn and Montevado. Later, on that day, the dam became the site of the first larger battle of the Swarm War.
- Gears of War 4 (First mentioned)