The Megacorpolis Accord

From this day forward, no longer will humanity put its fate in the hands of fickle patriots and intemperate politicians. The old world is dead, murdered by their mismanagement - a new world is born, a better world. A corporate world.
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— Marketing Ad for the Megacorpolis Accord

The Megacorpolis Accord is the treaty that saw the corporations first and greatest act of cooperation. With humanity nearing the brink of extinction from everything from nuclear war to climate change, nations started to crumble, fracture, and break apart. Entire continents descended into vicious fighting for resources and control, and that was really hurting the profit margains. That was no longer acceptable.
As things really started to go south, the richest corporations in the world came together in one grand concord to debate the future of the universe. Sure, the UN were still around and trying their best to keep things running, but some of the Corporations had a vision for the future... And a return to the old things of ways were not in that roadmap.
The Accord was the result of that meeting and that vision. In it, corporations agreed to construct a new home for humanity, a mega-city that could hold the survivors of the troubled times, and could pay for it. They agreed to put their resources towards ending and reversing the destruction of the planet, because financial analysts had found that the extinction of mankind and the end of the world would lead to a poor few financial quarters. And that was that.
When the nations were at their weakest, the corporations just withdrew their support and sales, and watched them crumble. Some tried to fight back, using their armed forces to seize corporate assets at gunpoint, only to find that half of their soldiers had already been put on corporate payroll, and the Corps had armies all of their own. A few nukes were thrown around, but none hit anything that the Corpoations care about, so life went on. Once things settled down and the Exodus ended, the Megacorpolis Accord was enshrined in the Megacorpolis center, even as the Corps started fighting with market share with renewed vigor and a heck of a lot fewer restrictions.