The outer colonies were overrun by a massive Saurian invasion and the survivors fled to the sector's capitol planet, Lenaion Eleusis, which was then targeted. To answer this threat, the Earth Union rallied the fleets of a hundred worlds to rendezvous in the Olmeios System and then the Castrum Gateway was attuned to its counterpart in Lenaion. Leading the way was Gungnir, the oldest warship of its kind still in service and the former flagship of the Earth Navy before the notorious Fafnir was built. Of course by this time Gungnir had undergone numerous refits and upgrades allowing it to serve as the command nexus for the entire defense operation against this new alien threat while Fafnir and another force was gathered to begin an offensive campaign.
"Defend with one arm while striking with the other, always make your opponent pay for showing you their intentions."
-UEG Marine Handbook, page 37