The U.S. before antitrust laws existed.
https://en.wikipedia.org/wiki/Standard_Oil
https://en.wikipedia.org/wiki/Standard_Oil
Carnegie Steel: Government intervention. The federal government imposed steep tariffs on imported steel, shielding domestic producers like Carnegie from foreign competition. Without those tariffs, cheaper British/German steel would have kept Carnegie’s dominance in check.
Southern Pacific Railroad: Government intervention. The federal government gifted it, through the Pacific Railroad Acts, millions of acres and subsidized loans. This state-sponsored land monopoly let it block competitors from critical routes.