The Arachnid War is the war between the JSDF and the Empire against the Alien bug-like Arachnids that invades the Special Region to consume its biomass and resources. It takes place after the international sabotage of the Gate.
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 weapons of the Imperial army are useless against the hard exoskeleton of the Arachnid. Moreover, despite having advanced weaponry and managing to kill a couple hundreds of Arachnid, the JSDF cannot handle swarm-like vast number, which was proving far too much for even firearms can handle.
After the Arachnid manages to slaughter thousands of citizen of the Empire as well as JSDF. Lelei was forced to close the Gate to prevent more calamity.