Anti-trust enforcement can take many forms -- breaking companies up is just one of the most extreme and obvious ones -- but often just the threat of action was

Anti-trust enforcement can take many forms -- breaking companies up is just one of the most extreme and obvious ones -- but often just the threat of action was enough since for a while the US government backed it up. A big reason Bell Labs was created and perpetuated was because AT&T feared being broken up. Bell Labs was effectively a PR vehicle they used to show the US government that they were giving back to the community, and an excuse to continue operating as a de-facto state-sanctioned monopoly. Today's toothless FTC and DoJ don't really inspire that kind of fear. Source (sort of, paraphrased) - This is a big theme of The Idea Factory which describes the heyday of Bell Labs. (Edited to further develop my thought)