The Interplanetary War, also known as the First Space War was a global and interplanetary conflict that lasted from August 2164 to June 2168, though hostilities ceased in December 2167, fought between the United Nations and the Congress of Solar Nations. All of the Solar System's nations were involved at some level in the war, making it the most widespread conflict at the time. Some 2 billion people perished in the conflict, most of them civilians.

The war came about after the sharp escalation of tensions between the UN and the CSN over the course of the previous decade, and was an extension of the UN-Lunar cold war that had been in effect since Lunarian independence. After the independence of several Asteroid Belt colonies and the moons of Saturn and Jupiter, the UN issued an ultimatum to the CSN that these new states were not to be acceded. They were, and the UN declared that a state of war existed between the two alliances on 10 August 2194. After a week of stalemate, the Lunar Republic seized and deorbited a colony habitat onto Earth, causing much destruction. The Martian space fleet attempted an orbital bombardment of Earth, but were repulsed. UN reprisals targeted orbital manufacturing centres, colony cylinders, and deployed an assault fleet to Mars, which launched a ground invasion. Once the Mars front was stabilised, the assault fleet continued on to a campaign in Jupiter and Saturn.

The stalemate resumed in Earth orbit until October 2195; the CSN deorbited six more colonies onto Earth, killing hundreds of millions of people and vaporising Buenos Aires. A subsequent orbital invasion of Earth, cooperating with various rebel movements, spurred several UN members to suspend membership and become neutral nations. Nevertheless, the Earth theatre became another stalemate as the invasion became a battle of attrition that the CSN invaders could not win. At the height of the invasion in December 2166, a division of orbital troops landed in New York City with the intent of capturing the UN Assembly and Security Council in a decapitation strike; but the CSN units were decisively defeated on Long Island after an encirclement campaign by American forces. A series of CSN defeats in 2166 and 2167 ended the invasion, and a string of UN victories in space forced the Lunar Republic to break from the CSN and pursue a separate peace. The CSN member states followed suit and an armistice was declared on 12 December 2167. The Algiers Peace Conference yielded a permanent peace treaty on 16 June 2168.

In the aftermath of the war, the Saturnine and Jovian rebel states were disbanded and their territories converted into UN Trust Territories; the Mars Democratic Union was replaced with the weaker Confederation of Mars; it, the Lunar Republic, United Asteroids, and the Republic of Caelum were made UN observer states with full membership being granted in 2200; territorial changes and indemnities were enacted; and an enormous relief effort was put in place for the hundreds of millions of displaced, injured, and bereaved persons on Earth and in space. Its biggest impact, however, was that the expanded UN began a concerted effort towards the colonisation of extraterrestrial worlds.

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