The Ostri Republic was a member of Union of Independent Republics that fought against the Coalition of Ordered Governments and their allies during the Pendulum Wars. Ostri was a coastal nation and enjoyed a lukewarm alliance with the larger and more aggressive nation of Pelles, another UIR member. Ostri also housed the Hammer of Dawn technology over which the Battle of Aspho Fields was fought. At some point between the end of the battle and E-Day, Ostri (and Pelles) became members of the COG. They were both led by Premier Deschenko when the Locust attacked, and nearly twenty brigades of troops were lost defending the nation. Eventually Ostri was abandoned to the Horde, and the populace moved to Pelles for their last stand. The nation was destroyed when the Hammer of Dawn was used as a last ditch attempt to destroy the Locust.
30 years before the Siege of Anvil Gate, Ostri was a sight of a major campaign. In the last years of the decades long war. Ostri was the center of the Hammer of Dawn project. If completed it would destroy the stalemate of the war and give the UIR orbital dominance over Sera. Until it was discovered by agent Louise Settile of COG intelligences leading to Operation Leveler. After the operations success, the COG was able to use the Hammer of Dawn and eventually the war came to an end and Ostri became a member of the COG.
Over a year after Emergence Day, Ostri was under the administration of Premier Yori Deschenko. Ostri, like all COG nations, was battling the Locust Horde, losing tens of thousands of Gears in the process. Deschenko retreated from Ostri and headed for Pelles. With the fall of Ostri it divided the Coalition and made a counterattack impossible. Though what remained of Ostri was wiped out of the face of the map along with all Locust held terroritories outside Tyrus during the Hammer of Dawn counterattack.