Landmines. Right now, they use a bare minimum of metal to evade metal detectors, so everything is mechanical. This would allow a processor and such to be in the mine so you could have it not detonate on friendlies, and deactivate itself/start beeping after a couple years. Heck, you could even have the mines networked so the whole field detonates simultaneously when it detects large number of troops are half-way across. I'm not sure if it's in the best interest of humanity to revisit this technology, but at least some of their tactical and humanitarian problems can be addressed.