The war began in 2357 when the Helghan Empire, under the control of Scolar Visari, sought to dramatically expand itself by invading Vekta, then invading Earth once Vekta was conquered, and destroying the UCN. Any opposition from the ISA-controlled systems would be crushed in a subsequent invasion of all other Human colonies.
The first stage in the Helghast's expansionist plans was the reclamation of the ISA controlled world of Vekta. Initially the ISA suffered massive casualties and territorial losses on Vekta after the Orbital Defence Systems 'malfunctioned', opening up the planet to a mass invasion force, and with a pair of high ranking double agents, the Helghast managed to take large swaths of territory and inflict severe ISA casualties in a savage blitzkrieg. However, due to the actions of Jan Templar and UCN reinforcements, the invasion was eventually halted and pushed back, and the few remaining Helghast of the 3rd Army were forced to fall back and regroup.Two years later, the Helghast homeworld of Helghan was invaded by the ISA in Nov. 2359 in an attempt to capture Visari and establish an easy to control puppet state. However, the ISA invasion has failed due fierce, unexpected resistance from the Helghast, culminating in the nuclear devastation of their own capital, killing the ISA soldiers attacking it, and in turn the bulk of the ground invasion force. Also, the destruction of many of their cruisers, including the flagship New Sun, and finally the death of the fleet commander, by a large counterrattack led by Colonel Radec has handed dominion of space and air to the Helghast, who can now bombard the remaining ISA forces and overwhelm the few remaining ISA cruisers . Finally, Scolar Visari, the ambitous emperor of the Helghast and the objective for the entire campaign, is gunned down by Rico before he could be taken into custody, causing the ISA to stage a frantic evacuation, which quickly devolved to a total rout, in a hurried and unsuccessful effort to withdraw from Helghan. Roughly six months after their defeat on Helghan, the Vektan Government capitulated to the Empire, formally ending the war in a status quo ante bellum as niether side could counter attack after that time, although the Helghast government played it off as the Empires' victory. However, conflict persisted between the Empire and ISA remnants on Helghan.
In the wake of Visari's death, a political conflict over who will assume the role of Autarch is waged between Admiral Orlock of the Helghast Military and Jorhan Stahl, influential chairman of the private weapons manufacturer, Stahl Arms. As the remnants of the ISA fought to survive, Stahl developed weapons using irradiated petrusite gathered from the Pyrrhus nuclear blast zone, such as a new Arc Cannon or irradiated petrusite-based perimeter defenses and energy shielding. In particular, an experimental flagship built by Stahl carried hundreds of missiles whose warheads were filled with irradiated petrusite and, alongside a massive Helghast invasion fleet, planned to eradicate all life on Earth by saturating the entire planet with the new petrusite missiles.
However, the political struggle between now-Autarch Orlock and Jorhan Stahl began to escalate, with successful operations conducted by the ISA remnant only making the situation worse. Eventually, the political conflict came to a climax aboard a Helghast space station, resulting in the death of Autarch Orlock and the Helghast race being plunged into civil war. This allowed the remaining ISA forces to destroy Stahl's cruiser with hijacked Helghast strike fighters, causing it to plummet to, then explode in low orbit over Helghan. The cruiser's destructive payload of irradiated petrusite missiles then detonated due to secondary explosions, with the resulting wave of energized radiation incinerating the entire Helghast invasion fleet, a large space station, and violently irradiating all life on Helghan. Managing to avoid the cataclysmic explosion in time, the remaining ISA troops onboard the hijacked strike fighters then made their way back home to Vekta.The Second Extrasolar War is now at an end. With the devastation of the Helghast species and complete destruction of their military, industry and leadership, the ISA have emerged as a pyrrhic victor, despite a near total loss of their military resources and personnel. The surviving Helghast and their remaining ships are given an escort to Vekta by the remaining Cruiser in the ISA fleet. After having relocated to Vekta, some Helghast exacted small amounts of revenge on the humans living on their half of the planet before the government decided to evict all non-helghast beings from their territory. The Helghast and human communities are then separated by a massive wall in Vekta City, tensions run high, putting the opposing factions in a Cold War scenario and setting up the events of Killzone: Shadow Fall.