The FTC blocks mergers all the time. Here’s some in the news:

https://khn.org/news/article/biden-ftc-block-hospital-merger...

https://www.airandspaceforces.com/ftc-blocks-merger-of-lockh...

https://www.nasdaq.com/articles/nvidia-and-arm-merger-deal-b...

https://www.housingwire.com/articles/50114-ftc-moves-to-bloc...

https://www.legalbettingonline.com/news/daily-fantasy-sports...

Mostly the FTC’s work happens quietly and unnoticed, but there are annual reports here:

https://www.ftc.gov/policy/reports/annual-competition-report...

And you can search merger actions here:

https://www.ftc.gov/enforcement/merger-review

Are there any significant historical cases? The Nvidia and Arm one seems like a big deal (if it succeeds), but the others just seem like random low-hanging fruit. I feel like the ones that matter (ISPs, cell phone carriers gobbling each other up, the Microsoft/Facebook/Adobes of the world, AMD buying ATI...) never really get stopped.

AT&T/T-Mo blocked:

https://www.justice.gov/opa/pr/justice-department-files-anti...

https://en.wikipedia.org/wiki/Attempted_purchase_of_T-Mobile...

T-Mo/Sprint would have been blocked (and fell apart multiple times) if not for the Trump administration:

https://en.wikipedia.org/wiki/Merger_of_Sprint_Corporation_a...

I think you’re undervaluing the significance of the FTC’s everyday work, but I’ll agree with your the Federal government has not been aggressive enough in exercising its anti-trust authority.

Isn't AT&T/T-Mo a DoJ effort, not the FTC? I don't see the FTC's involvement in your links... were they cooperating behind the scenes somehow?

The Sprint one was also DoJ, according to the wiki.

I get that the FTC on paper is supposed to enforce things like that, but their track record doesn't seem great. It just feels like a zombie agency that used to have teeth a long time ago...

Edit: I guess they've been squabbling for a while: https://www.cnbc.com/2019/09/18/the-ftc-and-doj-are-squabbli... but maybe are trying to work it out? https://www.ftc.gov/news-events/news/press-releases/2022/01/...

Then probably the next Republican president will come along and just got both their efforts all over again...

Ya the last big case was Microsoft. Antitrust is being revived after being mostly dormant for 40 years but Congress has never changed the laws executives and the judiciary have just chosen not to enforce them