Chikyuu Kaihou Gun ZAS

Sometime in the 21st century, humanity leaves Earth for Mars due to a legacy of environmental pollution on their home planet. Nobody inhabited the Earth until a group of super-intelligent robots from another galaxy suddenly took over. They have changed the once-proud planet into something that they can inhabit on. An impromptu liberation army was set up to save the planet from the robots; who somehow got rid of the pollution on Earth.