They killed the last two American troops who were attacked from the air (towards the end of the Korean war.)
That factoid from David Kilcullen's The Dragons and the Snakes: How the Rest Learned to Fight the West. I'm about 100 pages in, finding it interesting and convincing. So far he's using an ecological/evolutionary approach to the recent history (say from 1991 on) of war, and the changes in how the opposing parties have changed their tactics and strategies, mostly learning from defeats.
One observation is that NSA can gather much more data than they can analyze. Terrorist/insurgent organizations don't rely on privacy laws, but on hiding in the woods of all the other data. I think that also applies to the average citizen--we get lost in the mass of data, so we don't need to be paranoid.