It was the Third World War that was expected to be the end of things. Not the Fourth. Instead, the Third ended 'merely' with utter collapse of all of Earth's major states (due to economic disaster that the war turned out to be), leading to a series of conflicts such as the European Wars, Second American Civil War, Second Russian Civil War and, finally, the bloody end of the People's Republic of China in the world's first de facto nuclear civil war.
The Fourth World War has destroyed the first - and foremost - of human interstellar civilization (the first Terran Federation), by tearing out its still beating heart. The Federation was weak following the Third Colonial War (first all-out interstellar war, waged between the Federationist majority and the Democrat insurgents following their failed military coup in the name of restoration of democracy), however it could - theoretically - still fix its problems. Fourth World War ended it completely.
Ever since the Third World War ended, Unionists - essentially a post-leftwing movement fighting for the creation of one, centralized government for Earth - were slowly pushing towards unification. Against them was the disorganised mess of post-rightwing movements, including most of the remaining national governments, religious fundamentalists and nationalist groups (plust a lot of average people that saw the growing corruption of the post-UN Federation and saw the further dismantling of the nations as expansion of tyranny).
Last shreds of the Nationalists' support to the Federation was gone after the Third Colonial War. Democrats - that most of the Nationalists supported - lost. The last, desperate attempt to reform the Federation from an oligarchy led by unelected officials back into truly democratic organization has failed. And the victorious Federationists' political purges after the victory (done harshly enough to infuriate people but not enough to scare them into a compliance) has cemented the hatred. The war was at this point inevitable. The gossip about the Federation's plan to return to the now-forbidden surveillance via brain implants (made quite believable by the governmental paranoia) became the spark that fell onto a powderkeg.
Initial deployment was, de facto, an improvised military coup. Nationalist forces (mostly composed of remnants of national armies) planned to seize the headquarters of the Terran Federation (on Earth's surface) while simultaneously striking against its orbital installations. The plan both succeded and failed: the orbital installations were mostly taken down by a combination of boarding actions and earth-to-orbit missiles, and the headquarters were stormed. However, in the desperate battle against the Federation's reinforcements, most of the captured parliamentarians and officials died when their convoy was mistakenly hit by an orbital strike. This decapitated Federation to the point that there was nobody left to negotiate peace with.
Interstellar dominion of Mankind fell into a Fourth Colonial War (with remaining Democrats striking against outlying military bases, now seeking an end to the Federation instead of the reform). Earth, in the meantime, spiralled into a Fourth World War (also known as Fourth Terran War), as Unionists (Federation-sympathizers) raised against the Nationalists.
While Nationalists had greater numbers than Unionists, they lacked one very important thing. Political unity. This allowed the numerically inferior Unionists to succeed in deploying at least equal forces to every important battleground. Nationalists lacked centralized command until it was already too late to win.
From a geographical point of view, Middle East, Central-Eastern Europe and Africa were staunchly pro-Nationalist, Northern America, Far East save for China and Russia were equally divided, and Western Europe, Southern America, and China were pro-Unionist. The war started in 2151, but it was essentially over by 2155 once Northern America and Far East fell to the Unionists. The relentless (and bitter) defense of the remaining Nationalists prolonged the war until 2163. It was led to both sides employing various weapons forbidden by past treaties, including chemical and biological weapons.
In 2163, with remaining holdouts of the Nationalists in Central-Eastern Europe (Russia mostly fell), Middle East and Africa still relatively strong (even if without a chance for ultimately victory), they were (suddenly) cleansed with a mass deployment of nuclear weapons. Descendants of both sides accuse each other for it to this day: Nationalists see this as a much greater Hiroshima and Nagasaki done to force them to surrender (and lower their numbers). Unionists see this as a Nationalists' collective suicide in a hope of martyrdom and to smear the new Terran Federation's good name.
Collapse of Nationalists and the unification of Earth. More than a billion casualties, mostly civilian and due to final nuclear holocaust and biological warfare.
Great Schism of Mankind, absolute degree of hate and distrust between the Terran Federation (descendants of Unionists) and Outer Colonies (descendants of Nationalists), and the new Terran Federation descend into totalitarianism.