alan_cx has the best explanation that I've read on HN yet (though he's wrong about the "allies" part). It's essentially "trust, but verify". You might trust a b

alan_cx has the best explanation that I've read on HN yet (though he's wrong about the "allies" part). It's essentially "trust, but verify". You might trust a business partner to not screw you, but you should still read the contracts you sign with them. That's why we tell you to "get it in writing". Unfortunately, that kind of verification works because there's a legal system in place that you can invoke for punishment of contract-breaking, or at least compensation. At the nation-state level, that verification mechanism breaks down for several reasons. (1) There is no higher legal power to appeal to. The UN is a deal-broker, but it doesn't have the weight to throw around. (2) Things like the Great Firewall and propaganda will naturally make official news sources untrustworthy. You can see some of this on the stock market: most shareholders only have press releases and news reports to decide how the company is doing. But this is only half the story, which is why insider trading happens at all. So how do you get around these problems? By finding out directly in ways that are harder to fake. Which is what spying is all about.