Motor City Combine is what's left of the city of Detroit.
Already a tough city before the Great War, radiation, nukes, and craters only made it tougher. A lot of infrastructure survived however - infrastructure and know-how. They knew how to build, how to smelt, tool, mechanize, mass produce, and started turning scrap into trucks, guns, freezers, furnaces - you name it - and got rich selling it from the lakes to the bayous. Could have been the biggest city in the Midwest if it played its card right, but greed happened.
Greed and stupidity. The Buckeye Republic ordered a hundred tanks and a thousand guns from the Combine's biggest makers. All those fools could see were those pretty Buckeye bucks stacking up in their vaults. Weren't they surprised when those sod-kicking puritans took delivery of all those tanks and guns and drove 'em right back up the Erie shore to the MCC. The outcome of the war is not known and it's believed that it's still ongoing.