War broke out in the colonies first. It was over territory on Mars and Titan. Each of the colonist factions felt that they deserved more of the planet or moon for themselves. This was contrary to what the actual heads of each alliance thought. They thought the division of resources and territory was fair.
The fighting saw the first widespread use of infantry energy weapons rather than slug throwers. This helped make the fighting particularily deadly. At the end of World War III the US Alliance began using military grade lasers and rudimentary plasma cannons as naval weapons, but until the war in the colonies started no conflict had seen the use of infantry scale energy weapons and most medical facilities were unprepared for the kinds of injuries that these weapons inflicted.
The first shots were fired on Titan where the population had grown greater then Mars. The view of Saturn during the night was spectacular and could light up the night almost like day light. The fighting quickly spread to mars once word of the shooting had spread. The officials on Earth attempted to cover it up but all attempts failed. Eventually the fighting spread to Earth itself and it officially became known as the Fourth World War when European Union and Commintern started shooting at each other over no man’s land. The fighting was half hearted and no serious offensives were launched for three months before an armistice was officially signed. While the fighting ended quickly on Earth it continued out on the colonies because the colonists refused to accept the terms of peace which was a return to the status quo. It was clear that there would have to be some kind of intervention by the four alliances to stop this fighting. It would take years to get a military force to Titan and Mars but it was viewed as the best option at the time.
The United Nations took the lead rather than the four alliances. They assembled a force from the four alliances, built ships and then launched them at the two colonies and hopefully they would arrive before the colonies wiped each other out. They arrived at Mars eight months later to find that the colony had deployed Atomic, Biological, and Chemical weapons and the cities were largely destroyed. The forces landed and the fresh forces overran the remaining colonial forces. Titan was a different story. Four years the colony had been tearing itself apart. It did not deploy ABC weapons and the initial landing was heavily contested. US marines and Commonwealth Marines led the way. They landed first and were followed by the large groups of Commintern infantry and European Union armour. This was the first planetary assault in human history and it was not supported by warships in space. It was supported by fighters launched from carriers however. It took six years to pacify the colony with at least two reinforcement assaults from Earth.
While there is not much to tell about the war itself, the war led to significant changes to the way the Earth was run. The UN was now in control and it gave a voice to each nation within each alliance. The leader was voted in from the 250 odd nations that participated in the UN. Furthermore the UN decided that a new project must be launched in order to occupy the attention of the Colonies and Earth from future conflicts. It was decided that they would colonize the single planet detected by probes launched years ago that had arrived in the Alpha Centuari system. They could not get the probes close enough to the planet due to miscalculations in the trajectory of the probes. Never the less a mission would be sent.
At first developments in wormhole technology seemed promising. However they could not get a stable wormhole to form. The closest they came was at a research facility on Io. They managed to form a wormhole for three seconds but the wormhole destabilized for some inexplicable reason. An alternative technology came to light in 2256. It was called the inertial compensator which allowed a ship to accelerate and decelerate more freely. It soon became apparent that the light barrier could be broken and while this technology was not yet suitable for interstellar travel it had much room for improvement. In 2271 the light barrier was broken by a one man craft and the UN decided to begin assembling the mission to Alpha Centauri. Improvements to the technology continued while the necessary resources and manpower were calculated for two eventualities. If the world was inhabitable a colony would be established. If it wasn't the convoy would turn around and head for home. A convoy of 16 ships was assembled along with enough supplies to accomplish both missions. In 2285 the mission was launched and it passed Pluto only three days after departing Titan.
After two years of travel the ships decelerated from light speed inside the Alpha Centauri Kuiper belt. The convoy was scattered and gathered itself before moving on the planet. The four warships that had been sent along coordinated the convoy so that there would be an organized decent onto the planet. Once they arrived at the planet they discovered that it had a satellite network in orbit and contained an intelligent life form. The civilization appeared to be at a similar development to late 20th century Earth. The mission was to establish a colony so the convoy landed on one of the southern continents and established a small walled city in a relatively unoccupied area. First contact with the indigenous life forms was unsuccessful due to the language barrier. There were linguists on board but they could not speak the native language due to differences in anatomy. The written language was deciphered however and communication began. They called there world Artech and the Artechens were more than happy to allow the humans to stay in hopes of getting some of the advanced technologies that the humans used.
However the Artechens began falling ill. It was soon apparent that the Artechen immune system was incapable of recognizing the human viruses as pathogens and allowed them to run rampant in the Artechen body. Antivirals were ineffective because they neutralized the virus’s ability to mutate and allow the immune system to overcome the virus. What the humans needed to do was get the Artechen immune system to recognize the virus as a pathogen before administering antivirals. This effort was hampered when Artechen governments began demanding that the humans leave before it was too late, but it was already too late. The damage was done. It was also too late for the humans to simply pick up and go. Ships had been dismantled and resources consumed. The humans were staying.
Military units moved into the area and the colony was in jeopardy. The colony did possess weapons but no anti-tank weapons or anti-aircraft weapons. They did have the four warships in orbit however and both the humans and Artechens learned a new state of warfare, orbital superiority. The satellite network was wiped out over the colony, tank columns were destroyed when the moved towards the colony and even infantry units suffered at the hands of orbital bombardment. Aircraft were shot down before they could launch their payloads and missiles were even shot down. ICBM's launched at the colony were intercepted long before the colony was in danger. Attempts to shoot down the warships with ICBMs failed. The ships were hardened against radiation and that was all the atomic weapons could do in space was shower the ships with radiation. These were intercepted anyhow in case a weakness in the ships was discovered. With military intervention disastrously repulsed the Artechens resorted to isolation. This was unfortunate as the humans had discovered a way to immunize the Artechens. Within a year the planetary population had been devastated to the point of extinction. The planet was now solely in the hands of the humans. For 50 years the colony thrived. More settlements were constructed as more convoys arrived from Earth. The Artechen cities were left alone out of respect for the dead.