We already require extra authentication at a distance to arm nuclear weapons, and last season's 24 notwithstanding, we routinely operate military drones at a distance. Reportedly in the Falkland Islands war, Margaret Thatcher was able to extract codes to disable Argentina's Exocet missiles from the French. The simplest implementation might be like the proposal for land mines that expire after a certain time. Perhaps tanks — currently usable without even an ignition key — could require a renewal code digitally signed by the owning country to be entered manually or received by satellite every six months or so.
I'm a skeptic of kill switches, especially in consumer devices, but still found myself writing up the case for a way to disable military hardware in the field. There are lots of reasons it might not work — or work too well — but is there a way to improve on what we face now?