I suspect there’s an official definition of ‘act of war’ that they don’t fit. It’s also insane to start a war unless you absolutely have no choice.
Countries are constantly spying on each other, we do it to them, they do it to us. We let their diplomats into our country and they spy on us, and we know about it. The Soviet military used to have regular contact with US Army folks, knowing full-well they were watching them.
You cannot start a war over signals intelligence and espionage, you’d be fighting 100% of the time. Cyber attacks are just another form of these things.
The trick is to get real information out of the enemy without them knowing you got it. Those US soldiers were overtly spying on the Russians (they even had Russian ID cards, given to them by the Russians), but they were also doing more the Russians didn’t know about.
It’s a matter of picking your battles. Besides, allowing constant probing to be the norm means you have opportunity to get more information about them and, if you’re better than them, make their efforts wasted while learning what their capabilities are.
If I got to watch people try to break into my home without them knowing, I’d be much better prepared when they do launch a full-scale attempt.
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