Invader Demon's misson on Earth began during Operation Impending Doom I. He was given this planet, and sent off, before Zim ran havoc on Irk. He destroyed the records, and when they were being rewritten, the Tallests' secretaries forgot about Demon and Earth. They didn't exist anymore, as far as the Tallests were concerned.
While Zim was being banished, and Operation Impending Doom II was being planned, Invader Demon was preparing the Earth for conquest, not knowing any of this had happened. The first he heard about it was when his sister, Invader Eternal, crash landed on Earth a week before Zim arrived. Demon decided to continue with his mission nonetheless, to prove his worth to the Tallest.