It's been a while. Wikipedia [1] is relatively useful here:

1890 - Start of conventional anti-trust enforcement

1930 - Ramp up of law's usage (under FDR)

1966 - First dissent against anti-trust (Brown Shoe Co. v. United States)

1974 - First decision against anti-trust (United States v. General Dynamics Corp.)

1982 - United States v. AT&T allows break up of Ma Bell. Weakened enforcement allows re-merger.

1999 - Microsoft successfully fights off anti-trust enforcement prevent company from ever being split.

If you want anti-trust enforcement, do not elect Reagan and his descendants.

[1]: https://en.wikipedia.org/wiki/United_States_antitrust_law

It happens all the time.

But usually it’s blocking mergers and acquisitions. Here’s an example: https://www.nytimes.com/2022/11/21/books/penguin-random-hous...

Waiting to break up a company for anti-trust is like waiting for your house to completely flood instead of fixing the pipe before it gets to that point.

Not just huge companies! The entire venture capital model relies on providing a service substantially below cost and burning money to acquire users and eliminate competition so you can raise prices and profit later. In a sane world, Uber undercutting taxis with VC money should have been illegal.

Is it how it works in the EU? Has it worked well for them?

If the cost of initial operation were prohibited to be eaten by investment money, why the cost of development would not be, by the same logic?

I do think that there are cases of competition stifling through dumping, and that's illegal for a reason. Unfortunately, things are not as clearly delineated as with e.g. burning down your competitor's factory.