Tassilo Vago was among the first Zanscare commanders to lead the invasion of Earth. However, while BESPA was initiating a ground invasion, in space Tassilo was placed in charge of the construction and defence of the strategic attack satellite Keilas Guilie. His efforts initially proved successful as his fleet successfully held off the Federation Forces' Bagley Team while the final preparations for the Keilas Guilie were completed.
Tassilo intended to use the Keilas Gulie's powerful beam cannon to bombard the Earth and force the Earth Federation into submission. However, the satellite was never used against its intended target as the League Militaire, in tandem with the survivors of the Bagley Team launched an offensive against the Keilas Guilie. Using unorthodox tactics as well as its technologically superior mobile suit forces, the League Militaire was able to overwhelm and destroy most of Tassilo's fleet as well as gain possession of the Keilas Guilie itself.
During the battle, Tassilo Vago had been rescued by Cronicle Asher and was brought back to the Zanscare capital of Amelia, where he was subsequently sentenced to be executed by Kagatie due to the loss of the Keilas Guilie defense fleet as well as the satielite itself. Scheduled to be executed alongside the captured League Militaire pilots Uso Ewin and Odelo, Tassilo was able to escape the sentence during the League Militaire rescue operation. Shortly after this, the Keilas Guilie was used by the League Militaire to destroy a significant portion of the Zanscare's Sugan Space Fleet.
Tassilo reemerged later as the commander of the Angel Halo defense fleet. While Tassilo initially appeared to be loyal to Kagatie and the Zanscare leadership, he in fact never forgave Kagatie for scheduling his execution and sought revenge. Tassilo and several of his soldiers boarded the Angel Halo and were eventually able to take the Queen hostage.
Returning with the Queen to his flagship, the Amalthea-class battleship Shubaten, Tassilo attempted to escape with his forces and intended to assume control of the Zanscare Empire or create a new empire with Maria as the figurehead. His plans never came to fruition as the League Militaire pursued and eliminated most of his forces, including newtype ace pilot Fuala Griffon.
After the Shubaten itself was confronted by the Uso in his V2 Gundam, Tassilo executed Queen Maria after she implored Uso to destroy the ship. Tassilo then defiantly mocked the young pilot as being too soft, but Uso then used his Gundam's beam saber to vaporize Tassilo and the Shubaten bridge.
Tassilo Vago's actions had a large impact on the Zanscare war. His loss of the Keilas Guilie enabled the League Militaire to strike a crippling blow against the Zanscare space fleet. More importantly, his treacherous actions at the Angel Halo not only resulted in the destruction of key defense forces protecting the psycommu fortress, but the loss of the Zanscare Empire's nominal and spiritual leader Queen Maria Pure Armonia.