Complex sentence: It was during this phase of the war that the Iraqis made use of poison gas to repel Iranian attacks, both mustard gas and the more modern nerve gas were used demonstrating the Iraqi capability to manufacture and deploy such weapons of mass destruction.
Split into simpler sentences: It was during this phase of the war that the Iraqis made use of poison gas to repel Iranian attacks. Both mustard gas and the more modern nerve gas were used, demonstrating the Iraqis' capability to manufacture and deploy such weapons of mass destruction.