So, what are the ways in which intelligence, military and police organizations find out about terrorist plots?
I realize that there’s more than one way. Particular cases and general trends are both welcome.
Because like most criminals most are not not really masterminds and they make stupid mistakes?
Typically they contact an FBI informant (or are actively recruited by an informant) and the rest of the plot is basically constructed by the FBI:
Not counting people like the shoe bomber and the underwear bomber, who were caught at the last minute by airline staff, due to their incompetence, and the guy who (IIRC) tried to cross from Canada into the US with a bomb, and was caught by customs inspectors, are there actually many cases of terrorists who were genuine threats (as opposed to wannabes, talking to each other about it, but never really doing anything) who were thwarted by the intelligence services, or police or military? I can’t think of any offhand.
I’d say that in a number of cases in which terrorist plots have been thwarted, the exact circumstances are kept secret to this day, for instance during the “Troubles” in Northern Ireland. You get this impression when you read about incidents like the Loughgall Ambush in 1987 or the Coagh Ambush in 1991. There are speculations that the plans were uncovered either by an informer or by technical surveillance, but nobody knows for sure.
I don’t know the British laws, but I’m assuming that the relevant documents won’t be declassified many decades to come.
The same is probably true for many of Israel’s successes against Palestinian terrorists.
Security services routinely monitor ‘chatter’. On both sides of the Atlantic, phones, emails and social media are watched all the time for hints that anything is brewing.
A terrorist cell cannot really operate in total secrecy and isolation. They need money and supplies, a place to stay and information about likely targets. This exposes them to outside surveillance and subsequent disruption.
We have to assume that the security services are better equipped, better funded and at least as well motivated, as we depend on them to keep us safe. Of course they do fail sometimes, but I imagine they don’t tend to trumpet the successes too often.