During the Federation-Cardassian War, Thau worked as an engineer as a part of an engineering team setting up planetary defense systems outside the main city. As a neutral planet, Ubessi wanted to ensure its sovereignty, and engineering teams such as Thau's were used to install anti-starship weaponry systems. Thau and his team were on such a mission when Starfleet and Cardassian forces clashed over the planet during the Battle of Ubessi, sending a Starfleet starship crashing to Ubessi's surface, causing a massive matter-antimatter explosion, destroying most of the main city and polluting the planet. (Star Trek: The Prospect Chronicles: "What a Difference a Day Makes")
Thau's team survived, and, with the help of a severely-injured Ubessian military commander, evacuated to a military command bunker. Gathering a small handful of survivors, Thau emerged as a natural leader (after the military commander died of his wounds), contacting the Federation and Cardassians for assistance. The Federation were unable to render assistance (due to the distance of their front line to Ubessi), and the Cardassians refused to help.
Thau took his team into space via an Ubessian shuttlecraft to discover an abandoned Cardassian Galor-class starship in orbit. They repaired the starship, evacuated the survivors to it and set out to bring the civilian survivors to a safe haven. His plans went awry when the ship was caught in another Federation-Cardassian battle; this changed Thau.
After dropping off the civilians wanting to leave, Thau and his followers embarked on a mission to build up a powerful fleet.
By 2366, he and his followers had built up an impressive fleet, with more militant survivors joining their ranks. With this force behind him, Thau took on both the Federation and Cardassians in an attempt to reignite the Federation-Cardassian War. (Star Trek: The Prospect Chronicles: "What a Difference a Day Makes")