Because the GenTanks were bred specifically for war, it wasn't long before they - under the banner of 26th Century Neocron, led by Regant - fought against the longtime friend of the city's former leader, Ceres and his army of artificially intelligent, bipedal weapons platforms called Warbots.
The war, which lasted seven years, resulted in a condensed arms race between the two factions. In response to Ceres' Warbots, Regant and his loyal Psi Monks created a large number of hovering battletanks, each carrying a GenTank pilot. In response to these "hovertanks", Ceres' scientists took the inevitable path and developed nuclear weaponry, based upon the atomic bombs first dropped in the 1940s.
Because Ceres' forces are entirely mechanical - and shielded from the intense radiation - his Warbots are completely immune to the nuclear weapon he developed. After detonating such a device in the Wastelands and seeing their forces annihilated, Regant - rather than surrender - turned to his Psi Monks who, through their psychic espionage, had managed to emulate the radiation shielding technology.
After adapting the shielding to suits of durable armour, another batch of Gentanks was bred and, after being housed inside the armour, were sent into battle. With Ceres' threats of detonating a second nuclear weapon over Neocron now rendered irrelevant, Regant's army was able to invade the Dome of York unhindered.
Following the war, having been denied access to the holodisks which shared his ally-turned-enemy's name, Regant decided to publicly execute Ceres and take the holodisks for himself, which in turn led to the 22nd Century technology becoming available to Neocron as a whole, allowing it to grow into the city it is today.
Rather than be exterminated and disposed of like outdated weapons, the surviving GenTanks were, rather surprisingly, spared and left to their own devices after the program which created them was discontinued. For much of the remainder of the 26th Century, they lived in a state of self-exile, remaining away from their creators.
However, over time, the GenTanks began to integrate with society. While never accepted into Neocron's police force, the GenTanks saw their genetic upbringing more than suitable for private security, leading many of them to take jobs as bodyguards, bouncers and the like. While the original GenTanks have long since died out, their descendants are still among the city's population.