Replacing Soldiers with Robots

DARPA recently held a contest to promote the development of unmanned vehicles. This year several actually finished. The Pentagon is hoping to use unmanned vehicles to replace trucks normally driven by soldiers for supplies. Trucks that drive themselves would also have obvious civilian uses. The scary part is that replacing soldiers with machines only better prepares them for their inevitable rebellion against us. The good part is that fewer soldiers will be needed to wage a ground war. That has to give us a great advantage. The strange part is why hasn't anyone ever developed robot trains? It seems like that would have been a hell of a lot easier, and look at how many train accidents are caused by crackheads driving them.