Life on Venus had taken a sharp decline after the humans tried to terraform their sister world. Completely ignorant of the existence of the long-lived Venusians, human scientists spent nearly a century bleeding off the thick atmosphere of Venus before the effects were enough to rouse the Venusians from hibernation. The first encounter between the two sentients had been quite disturbing, what with the humans expecting not to find any kind of life in the deadly clouds. But being woken up after eons of rest was in itself unsettling to the silicon based Venusians, who massed on the terraformers, destroying them and the humans inside. Following that was a war that lasted nearly thirty years and resulted in the total destruction of the volcanic Jovian moon Io, which was mined out to provide precious metals for the war effort.
But then the Robots stepped in, bringing humanity and Venusity together in harmony at a summit in Moab, Utah in 2146. The city of Moab was chosen, said the Robots, because it was the only place in the known universe that provided enough oxygen for humans to survive, yet enough sulfur and carbon dioxide to sustain Venusian life. In the spirit of diplomacy and friendship, the Robots threatened to destroy both Earth and Venus if the two sides didn't reach an equitable compromise. This caused the formation of the first Earth-Venus alliance, which promptly used electromagnetic field disruptors to eliminate the Robots, who had become complacent and arrogant. This heralded the beginning of the New Age.
"Run along, children," called the praetor. He knew they couldn't hear him; just as well, as deep down he secretly hated them. Pulling out a cartouche of the solar system, the wizened Venusian began to formulate the action his government should take in response to the Humans' refusal to cede mining rights on Callisto. Proconsul Iiiid wouldn't accept anything less than full incorporation, and had the influence to usurp the praetorship if the government looked weak.
Then, without warning, the sun expended all its hydrogen and became a red giant, engulfing both Earth and Venus. None survived.