The Krimzon Guard are the main Army of Haven City and were under the command of Erol until he was killed by Jak and once Torn. They are very loyal to Baron Praxis and will do any task the Baron tells them to do.
The Metal Heads are a vicious alien race that invaded the planet to get the precursor stone. Their leader is Metal Kor. There are many different types mostly designed for Combat.
After Jak, Daxter, Keira, and Samos opened the rift gate, the Metal Heads attacked the planet as it had large amounts of precursor technology present. They swarmed across the planet, destroying most of the villages in the world.
Then some time later the great Hero Mar built Haven City and the Shield Wall to protect the people and the Precursor Stone from the Metal Heads. The Metal Heads then made an impenetrable nest where Metal Kor plotted against the city. The Metal Heads attempted to attack the city many times, but were repelled by the shield wall. Eventually the threat became too great and Mar built a weapon powered by the Precursor stone to break open the Metal Head nest. Unfortunately he "died" before he had the chance to use it. During the middle of the war Damas came to the throne but was betrayed by Baron Praxis and banished to the Wasteland where he either founded or came into power over Spargus. Praxis's campaign was more destructive for Haven, using propaganda and fear of the Metal Heads to keep the populace under his rule. He once led a failed attack on the Metal Head Nest losing his right eye. The remains of tanks and other machinery litter the nest. The Metal Heads launched a more successful counter attack, taking the section that became known as Dead Town. Baron Praxis was desperate and captured Jak who had just arrived from the past and other people to use in his Dark Warrior program which involved injecting toxic Dark Eco into the blood stream. All died except Jak. 2 years later, the Metal Heads attempt to sabotage Haven city using small, insectoid versions of themselves referred to as "Metal Bugs". The bugs were lead by Kaeden, Kor's second in command. Daxter fights and defeats Kaeden, ending the Metal Bug threat. Daxter then rescued Jak and joined the Underground.
The Underground sent Jak on many Missions against Baron Praxis and the Metal Heads until Jak finds out that Baron Praxis is bribing the Metal Head Leader with Eco to only attack the city just enough to ensure he stays in power. After Jak discovers Mar's Tomb and takes the two tests of Manhood to get the Precursor Stone, Baron Praxis shows up in his Spider Mech to claim the Precursor Stone. Despite Jak's best efforts, Baron Praxis gets to the Precursor stone first and escapes.
Back at the Underground Torn explains that Baron Praxis threatened to kill Ashelin if he didn't give the location for Mar's Tomb. Later after getting the Life Seed from Dead Town, it is found out that the Baron plans to crack open the Precursor Stone somehow but this would destroy the world. Jak goes to the Weapons Factory and after fighting his way to the top he meets Krew who is building a Piercer bomb which he plans to put the Precursor stone in and hide it in the last shipment of Eco and deliver it to the Metal Head Nest. Jak knows what that would do and kills Krew after fighting him and his clones and escapes before the Piercer bomb explodes. But a communication from Vin reveals that Metal Heads have invaded the city and are overwhelming the Krimzon Guards and also that Kor deactivated the Shield wall, and is immediately cut off as Metal Heads invade the Power Room.
Meeting at the city construction site, Jak confronts both Praxis, and is surprised to see Kor there. Kor tells Jak that in his darkest nightmares, he already knows. To everyone's horror, Kor takes on his true form (known as Metal Kor), the Metal Head Leader. Kor demands the Precursor Stone, but Praxis refuses. Kor strikes him and his guards with an eco blast and sends them flying across the area. After Kor completes his transformation and flies off, Praxis shows the two a second Piercer Bomb which hides the Precursor Stone. He then dies, and Daxter recovers the Stone.
Heading to the Metal Head Nest Jak and Daxter discover Metal Kor and after a climatic battle, Kor tries to depart through the Rift Ring but Kor crashes into it, severing his head and Damaging the Ring. With Metal Kor, Baron Praxis, and the retreating Metal Head Army defeated, The Krimzon Guard are victorious.
The war left the city crippled and its Army depleted. Ashelin became the Governor of Haven City and Torn became the Commander of the New Krimzon Guard (Later the Freedom League). The Grand Council was established led by Count Veger. On the Metal Head front, the death of Kor scattered and threw the Metal Heads into chaos resulting in only the strongest surviving.
Casualties include a large part of The Krimzon Guard and Metal Head Armies and the world would enter a new age called the New World. However, peace did not follow the war, and the Metal Heads later invaded the city a second time and Haven City lost control of the KG Death Bots resulting in the War for Haven City which would be just as destructive.