The 30 second answer is that standard oil owned an incredible percentage of the oil market, both in horizontal expanse as well as vertical integration.

They were broken up by the government through antitrust regulations, and IMO the most interesting part is the constituent parts became worth quite a bit more after breakup.

Curious about how anti-trust laws for an e-commerce firm might play out in 5-10 years, with the internet + globalization having come so far since Microsoft, I'd imagine even the US government would start to lose leverage breaking up something like Amazon.