Complex sentence: The Forerunners soon fought the Flood themselves; after exhausting every other strategic option available to them the Forerunners activated weapons of mass destruction known as the Halo Array, killing all unprotected sentient life in the galaxy to deprive the Flood of their food.
Split into simpler sentences: The Forerunners soon fought the Flood themselves; after exhausting every other strategic option available to them, the Forerunners activated weapons of mass destruction known as the Halo Array. The Array's firing killed all sentient life in the galaxy to deprive the Flood of their food.