It's worth pointing out that cops in the US adapted to these problems through using grand juries to cast wide nets and do punitive fishing expeditions in the wake of any serious suspected left-wing actions.
Someone starts a fire at an army draft office.
The cops look through their files for anyone who might be in the political orbit of someone who'd want to do that. People picked up at protests, for drug charges, vandalism, anyone who is already on their radar. They look into their known associates, anyone they live with, anyone they drink with.
Then they start subpoenaing these people for a grand jury summons. They give you immunity (but only for the matter of the grand jury!) so that you can't exercise the fifth amendment against self incrimination. If you say nothing, they can imprison you almost indefinitely for contempt of court. If they catch you in a lie, that's criminal perjury.
They'll ask you for information on everyone you know. Obviously they'll ask about their involvement in any crimes, but they're casting a wide net. Who knows who, where do they hang out, who talked to who about what and when. They'll ask you to spill interpersonal stuff, whether anyone is cheating on someone, whether people have substance abuse problems or embarrassing personal issues, if anyone is closeted, anything they consider dirt. Anyone you name is gonna get subpoenaed and they'll be asked for all this information on everyone they know, including you, and although you have immunity from your own testimony, you don't get immunity from each others.
Assuming you didn't personally do anything they can prosecute you for in the matter of the grand jury, they'll go after you based on what they know. The cops will arrest you on any little thing they have suspicion of, even if they know they can't prosecute you, just so that they can keep you in jail for a few days while you miss two shifts at work and your friends have to scramble to raise bail. They'll leak any embarrassing info that comes out, to your boss or your family or even the local press. Whatever they can do to make your life a little harder.
They will lean fucking hard on anyone who is involved in the scene but had second thoughts or felt like they were dragged into something they never wanted to do in the first place by their friends. The cops will say 'do you want to get your life ruined by people who did something stupid over something you barely even believe in' and sometimes that's a very compelling argument! If people have dependents or kids who they think won't be looked after if they go to prison, there's a lot of pressure to cooperate.
It's important to note two things:
1) Based on the ratio of actual prison sentences to maximum possible sentences for the charges, it's better for people not to cooperate with the jury both individually and as a group. People who talk still get sentenced, with the information they helped provide.
2) These aren't surgical strikes, they're an artillery barrage designed to destroy infrastructure and send people running for cover. Cops don't want you to have friends, they don't want you to hang out and have fun, they don't want you getting or providing food or shelter through anything you can't get fired from. They don't want committed direct-action people swimming freely through a sea of friendly people. They're not scared of the flower, they're scared of the soil that grows it.
