Interplanetary Strategic Alliance

The Interplanetary Strategic Alliance (ISA) served as protagonists throughout most of the Killzone series, meant to protect the colonies of the United Colonial Nations (UCN) and enforce the UCN's will by any means.

Inauguration of the ISA
As new interplanetary colonies grew, the UCN discovered the lack of effective long-range communication caused gaps in the ability for the UCN to provide efficient defense throughout their star systems. To combat the lag, the UCN organized the Interplanetary Strategic Alliance (ISA), a body that allowed individual colonies the ability to integrate defensive capabilities to their specific requirements as well as enjoy full partnership in a unified defensive alliance of multiple colonies.

Localized colonial administration provides funding and manpower whilst the UCN provides the training, equipment, and full political support for all of the interplanetary colonial administrations. The ISA acts as the main policing and defensive body for the citizens, and the main revenue gathering party for the local government. Each ISA command is directly subordinate to local colonial governmental authority, which in turn is subordinate to the UCN interplanetary authority. Since each ISA Command is an independent unit of each colony, it is not linked to the military command of the UCN. As a result, the ISA Command's budget and strength are determined by the funds each local colonial administration provides.

By 2133, ISA Altair and ISA Alpha Centauri were estblished.

The Second Extrasolar War
Following the defeat of the Helghan Corporation in the First Extrasolar War, ISA Alpha Centauri was given direct control over Vekta, thanks to it being the strongest and best trained ISA Command.

ISA Alpha Centauri was the first command to wage war with the Helghan Empire following Scolar Visari's invasion of Vekta, beginning the Second Extrasolar War. The ISA was able to repel the invasion of Vekta, but lost a nuclear weapon to the retreating Helghast forces. To regain the lost nuclear asset, as well as forcibly conquer and ensure that the Helghast would never invade another planet again, ISA Alpha Centauri planned a retaliatory invasion of Helghan in order to capture the warmongering ruler of the Helghast, Autarch Visari himself. While the first wave of the invasion was completely wiped out, the second wave made far more progress and eventually stood poised to advance through the capital city of Pyrrhus and into Visari Palace itself. However, the rogue nuclear weapon Red Dust was detonated over Pyrrhus, annihilating all ISA and Helghast forces that remained in the capital city. Further efforts at victory were hampered by the execution of Visari by Sergeant Rico Velasquez, who defied direct orders of capturing the dictator alive. The remaining ISA forces then had to contend with a massive Helghast counterattack determined to avenge their martyred leader,  nearly overruning the remaining ISA ground and space forces.

Eventually, after a long period of being cut-off from reinforcements, the ISA remnant managed to escape Helghan after boarding a Helghast space station and hijacking several Helghast strike fighters. Fighting to escape, they used the fighters to destroy the experimental cruiser of Helghast leader Jorhan Stahl, who planned to bombard Earth with hundreds of irradiated petrusite warheads, thus killing or irradiating millions of people on the UCN capital world. The experimental cruiser's destructive payload was instead detonated over Helghan, irradiating the entire planet and obliterating the entire Helghast invasion fleet destined for Earth, along with the surface of the planet and the majority of its population. Avoiding the cataclysmic explosion in time, the remaining ISA troops aboard the hijacked Helghast strike fighters made their way back to their home colony of Vekta.

Technology
Since ISA forces vary by the colonies they protect, the quality of technology within them differs. ISA Alpha Centauri for example heavily relied on static planetary defense grids and localized security. During the invasion of the First Extrasolar War, the Helghast overwhelmed the colony as a result. However, as the war progressed and more aid came from the UCN, ISA Alpha Centauri became a more powerful force. Powerful cruisers, such as the New Sun, indicated a radical swing toward offensive might.

ISA Navy
This branch maintains and uses all aerial and space fleet assets, including Intruder dropships, ISA VTOLs, ISA Heavy VTOLs , and space cruisers.

ISA Marines
The Marines provide a shock-force for ISA invasions, being the first troops in any invasion. Marines formed the core of Vekta's counterattack against Helghast, and saw the most combat. Despite the heavy casualities sustained, they survived long enough to wreak havoc and return home. Eventually, with the Helghast migration to Vekta, the Marines begin to provide security for ISA installations.

Rapid Reaction Force

The first line of defense against invaders or internal enemies, the RRF is also a riot control department and helps to control riot situations in Vekta. They maintain a gendermarie esque role, policing the colonies in peace time, and serving as front-line troops in war. Unfortunately, at the start of the Second Extrasolar War, they were easily brushed aside until remnants of the Force held out long enough for UCN reinforcements to arrive. When the Vektans would invade Helghast, RRF veterans formed the core of ISA Legionnaires.

VSA

The Vektan Security Agency(VSA) is the security branch which provides law enforcement in Vekta City and other cities. After the events of Killzone 3 the VSA is led by Shadow Marshal Thomas Sinclair and who doubles as the Head of Shadow Marshals. This branch also consists of the VSA Special Forces and perform reconnasience missions beyond The Wall (the border between New Helghan and Vekta).

Weaponry
The ISA produces all of its weapons, combat vehicles, and equipment. These are all developed by different branches of the ISA manufacturing brand, including (but not limited to):
 * ISA Laboratories: producers of the M327 Grenade Launcher.M82_Assault_Rifle.jpg


 * ISA Weapon Systems: producers of the M82 Assault Rifle and its sucessor, the M55 Rumbler.

The facilities used to manufacture all ISA-brand equipment are likely located on planet Vekta.
 * ISA Vekta Shipbuilding Division: producers of the ISA Cruiser.

Trivia

 * In Killzone 2 multiplayer, when playing as an ISA soldier, the HUD is displayed in a light-blue color unlike the campaign.
 * The ISA logo looks highly similar to a commonly-used symbol in other games. It most resembles a sideways version of the symbol from the Marathon game series or the Tau symbol from the Warhammer 40,000 franchise. It most likely represents the relationship between Earth and the colonies, imaged by how the moon "revolves around the Earth".
 * The ISA logo looks incredibly similar to the logo of the 91st Reconissance Corps from Star Wars, albiet flipped on its side.