The Arachnid War is the war between the JSDF and the Empire against the Alien bug that invades the Special Region to consume its biomass and resources. After the Gate opens to the homeworld of the Arachnid, the Hive Mind of the Arachnid commands its Warrior Bug to attack the Special Region in which all primitive weapon of the Imperial army are useless against the hard exo-skeleton of the Arachnid. Moreover, despite having advanced weaponry and managing to kills a couple hundreds of Arachnid, the JSDF cannot handle swarm-like vast number, which is prove far too much for even firearms can handle.
After the Archnid manages to slaughter thousands of citizen of the Empire as well as JSDF. Lelei forces to close the Gate to prevent more calamity.