Kidou Senshi Gundam: Uchuu no Ixtab

Universal Century 0079. The Magellan-class battleship Toutatis tracked a Musai-class ship on its way towards Side 2 for a preemptive attack following the Duchy of Zeon's declaration of war. Eventually they crash-landed on the Island Iffish colony and what they saw there was a landscape wiped out by poison gas, realizing Zeon's aim to drop the colony on the Earth. The Toutatis, equipped with a nuclear warhead, attempts to destroy the colony.
Meanwhile, a Zaku platoon had entered the colony by chance, ordered to subdue and silence dissent among friendly forces.
(Source: Gundam Wikia)