Bob Lambert https://en.wikipedia.org/wiki/Bob_Lambert_(undercover_police... was one of the people responsible for the 'McLibel' leaflet that turned into the UK's longest running libel case.

These days it looks to me like Extinction Rebellion (XR) are being run by spies, since all their activities are so counter-productive in terms of making people hate environmentalism.

> I seem to remember a story of a mafia don who turned out to be an informant, which seems wild to me.

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

The older you get, the more you've got to lose, and a lot of people become less radical with age.

At the same time you also tend to be a higher value target for external actors compared to young, new, members of an organisation. So in the young end of it you'd prefer infiltration to recruiting informants, in part because young informants also tend to be less reliable than old, for the reasons above. If it's your agent loyalty isn't as brittle as when you've convinced someone to become a traitor.

When news of government or other external actors having gained access breaks, you typically don't want your infiltrators to become known if you can avoid it. It's different in some settings, antifascists commonly do the opposite and try to protect their informants so they can keep working if they move on to another far right group, while their infiltrators sometimes go public with what they've done.

They are the highest value targets who coordinate the group's activities. Most will have done something at some point that could land them in prison for a long time; this is how they are typically turned. The ones that haven't done anything the feds can feasibly threaten them with often get bribed, which is where things can get really messy because you end up with policing organizations essentially permitting organized crime under a series of conditions (e.g. keep things quiet and we won't bother you). There are many documented instances of this happening on the street level, and some evidence to suggest it is occurring at the national level and the international level. It can be counter-intuitive to think of these organizations being so easily compromised, but a good rule of thumb is that no organization dedicated to criminal or subversive activity is going to evade the attention of the feds, and once that attention is there, they will almost never be able to outspend them. It's comparable to a small business trying to harden their systems against a state actor.

> I seem to remember a story of a mafia don who turned out to be an informant, which seems wild to me.

There's also "Jimmy the Weasel" Fratianno who turned FBI informant. In Ovid Demaris' book "The Last Mafioso: The Treacherous World of Jimmy Fratianno", he quotes Jimmy even laughs about being one, taking money from FBI and still running his rackets for a time.

If you get enough on them that you could probably prosecute them successfully, then you have enough on them to try to turn them. And if they're the only one at that level that you have enough on, rather than make one prosecution, it may be better to turn them.