Short, ELI5 answer: Spies.
For agencies like the CIA and NSA in the U.S. or MI6 in the UK, this is basically what they do all day: find out what other countries are up to. It can be as exciting as bribing a foreign official to covertly send you highly classified information or as boring as reading the local newspaper and seeing what it says. You might get the information directly, or you might have to piece it together from clues. Either way, the international intelligence community generally doesn’t like surprises, and therefore throws a ton of resources into knowing what everyone else is up to all of the time.
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