Complex sentence: The Earth attack force arms nuclear devices on Mars and departs just before Mars is destroyed in the predicted cataclysm, ensuring the peace and safety of Earth as the Martian race has been doomed to extinction.
Split into simpler sentences: The Earth attack force subsequently place armed nuclear devices on Mars and departs just before Mars is destroyed in the predicted cataclysm. Thus ensuring the peace and safety of Earth as the Martian race is doomed to extinction.