There was no civil war1 to precipitate the transfer of power from the governments to the corporations. The transition happened gradually. Government increasingly granted power to corporations until it was unable to stop their abuses. The corporations started behaving like governments; creating agreements with or sanctions against remnants of foreign powers or with other companies, levying taxes on the populations under their control. Their most obvious step towards statehood was the hiring of freelance mechanized armies. The mech armies could be trusted right up to the minute their contract expired. They were fiercely loyal to the current employer, and could be counted on without question, but once the contact expired that loyalty was up for sale.
Society devolved. Because the corporations and the mech armies looked out only for themselves, no one looked after the needs of the public. Roads not in use by the corporations fell into disrepair. Interplanetary travel stopped for everyone except corporates who used their company's cruisers, or mechs who could provide their own. Jails, courts, police, the entire justice apparatus simply ceased to exist. Populations were left to fend for themselves. Some people banded together, working in ways similar to pre-historic native Earth cultures, dividing labor between those who could make/repair, those who could hunt/gather, and those who could farm. These groups grew insular, viewing outsiders as a potential threat to their precious resources. Others became nomads, following herds of wild animals, or simply preferring solitude to the inevitable politics involved with a group of humans. These were distrusted by any community they came into contact with. Corporates referred to anyone making it on their own - either community or nomad - as a wildling.
Corporations typically left the wildlings to themselves, except for when they needed laborers for factories or plantations. When the need arose, they'd commission a mech troop to round up people like so many horses. Obviously, wildlings became mistrustful of mech pilots, even when they meant no harm. On the other hand, they couldn't help but envy the easy lifestyle mechs seemed to live.
Corporations stopped competing in terms of market share, and started less abstract contentions. A corporation hired freelance troops to protect its interest, in say an oil field or a thorium mine. Eventually, maps of the system became similar to Earth's colonial counterparts, when European nations held territories all over the globe, each subject to its conqueror. Mech troops were typically paid to ensure that there was a local population capable of doing the hard manual labor required by the nature of the territory, usually working a mine or factory. These laborers were de facto serfs. They had no hope for freedom, owned 'only their bellies', and relied on benevolent mech troops for protection from predatory wildlings.
This is the story of how I change all that.