The fiefdoms I'm thinking of were created naturally inside the organisation, not acquired. Somebody's power grows, a department with ten people becomes a business unit with a thousand people due to its success, other managers resent that, and senior management mistakes this as an opportunity for "internal competition" rather than a dysfunction which needs to be crushed, if necessary by terminating those who promote it.

Lots of things are hard about M&A but it's made easier if people remember from the outset that their business exists in the world. You don't have and can't build a business which functions independently from the world, stop trying. On the technology side that means things like using FQDNs in URLs (https://webinar.mycorp.example/2020-New-Products not https://webinar/2020-New-Products)