Interplanetary Strategic Alliance

The Interplanetary Stratgeic Alliance (ISA for short) is the main protagonist faction in the Killzone series and is the military police of the United Colonial Nations and enforces its rule over the colonies. Despite being the protagonists however, the ISA is critized by many Killzone fans of being the "True" villain in the Killzone Franchise despite the warmongering Helghan Empire and blame them for the Helghasts rise to power.

History
The ISA was founded in 2133 in response to the lack of contact that the UCN had with the other human colonies outside of the Solar System. Following many insurrections from human colonists to resist UCN rule, the UCN had created the ISA in an effort to protect the Colonies and enforce the UCN's rule. This lead to other bitter resentments and soon it spilled over when in 2199 the Helghan Corporation launched an insurgency against the UCN and the ISA to acheive freedom from UCN rule starting the First Extrasolar War. The war ended in 2204 and soon all Helghan Corporation colonists fled to Helghan and the ISA was ordered to blockade the planet by UCN orders and prevented any form of help from reaching the planet causing the rise of Fascist dictator Scolar Visari and the establishment of the Helghan Empire in 2347.

Second Extrasolar War
ten years after the rise of Visari, the ISA planet of Vekta had tried to reach diplomatic peace with Visari but failed miserably and then in 2357 the Helghast Military now newly rebuilt and ready for war had launched a massive invasion of Vekta and started the Second Extrasolar War. The invasion had began when ISA General Stuart Adams and Dwight Stratson had disabled the Orbital Defense Platforms and allowed the Helghast Army to invade and ravage Vekta. Helghast troops lead by General Armin Metrac had devestated southern Vekta and slaughtered untold Vektan civilians. in 2359 the Helghast Forces had finally managed to retreat but were soon forced back into war when the ISA had began to invade Vekta in Operation:Archangel

Invasion of Helghan
In 2359 the ISA Army had amassed large armies of troops and were sent to Helghan during Operation Archangel to remove the Helghast ruler Visari from power following the end of the war in Vekta. ISA Forces were ready to go when Visari made a statement ready to obliterate all who dare attack the homewolrd of the Helghasts. Within hours, the Invasion Fleet was torn apart as Anti-Air defenses on the planet's surface had devestated the cruisers forcing the ISA Marines to land and suffere heavy casualties upon landing.

Trivia
Many consider the ISA to be the main nemey for the follwoing reasons:
 * The ISA had invaded and colonized Vekta when the Helghan Corporation resisted the UCN
 * The UCN had made the ISA blockade Helghan to enforce their rule