The Battle of the Oort Perimeter
The Battle of the Oort Perimeter is a pivotal event in Earthfleet history that involved a great battle between the Varnok Black Fleet and Earthfleet, with Sevok's personal Gayik'Von combat fleet entering the battle late in the fight, but ultimately playing a major roll in the final, ultimate defeat of Mobeic and the Black Fleet. The battle began with a surprise attack by the Black Fleet, who was sent by Phillip Augustus to attack Earthfleet and exterminate all humans from the galaxy. Upon arriving they encountered the Oort Perimeter, a massive defensive shield grid that protected the Sol Space and was Earthfleet's first line of protection against attack. At first the shield held, just as designed, and prevented the fight from escalating any further. However, it wasn't long before a dangerous flaw in the shield, an energy wave generated by the shield's own operation, was discovered that risked opening up a massive breach in the shield and allowing the Black Fleet to enter Sol System uncontested.
To prevent this, a much smaller hole was opened to dissipate the wave and save the shield. However, Earthfleet was unable to reinitialize the shield and close the gap before Mobeic's fleet took advantage of the breach. As a result, Earthfleet was forced to fight a costly war of attrition against Mobeic and his forces, leading to a large portion of Earthfleet's main battle fleets being wiped out in the fight. The fleet bravely held the Varnok back, and kept them from breaching their defensive lines. However, as the battle progressed, the fleet soon found itself at the point of collapse and was strongly contemplating falling back to the Kuiper Perimeter in hopes of making a last stand there against the Varnok while at the same time praying for help to arrive that would save them from almost certain defeat. Thankfully, before they were forced to make such a difficult decision, Admiral Sevok of the Gayik'Von arrived and forced the Black Fleet to withdraw from their engagement with Earthfleet, in an effort to fend off his fleet. However, due to their already weakened state, and Sevok's superior strategy, the Black Fleet was swiftly defeated, and ultimately forced to surrender after the destruction of Mobeic's command ship, and him aboard it as well.
The aftermath of this battle left Earthfleet in a very sad state that took them many, many years to recover from. It also, later on, forced the complete rethinking of the defensive shield grids surrounding Sol System, and their eventual reworking into a better, more efficient, and more effective five layer system of defense, vs the previous two layer, which created a new and much improved protective bubble around Earth and the entire inner solar system. This battle also saw the end of the larger battleships and dreadnoughts of prior Earthfleet history, leading to the rise of a more powerful, efficient and effective battle force consisting of mostly Shipkillers, backed up by a handful of specialized super ships, such as the Mythos class. This change in doctrine, brought about by this battle, also aided Earthfleet in fending off attacks from the invading forces of the Marargon, as well as successfully handling several other major and pivotal events in Earthfleet history. Had this battle not happened, its possible that Earthfleet might have come out the worse for it in some of those later battles, or could've potentially been wiped out completely.
This battle also set in motion events that would eventually lead to the discovery and defeat of the Crassians, a malevolent and blood thirsty race who sought the conquest of the entire galaxy, and the extermination of all humans. Thus, while tragic, this battle was an important turning point in Earthfleet history, and one that brought about great and important changes, both in the fleet, and the galaxy as a whole.