Minefields aren’t ever really ethical, even if they are defending your family from some foreign army. Today’s invention is a new way to lay mines which is somewhat less horrendous than normal.
A robot device is programmed to traverse a stretch of territory, pressing into the ground small, bullet-like mines (designed to hold up an attack, by inflicting minimal wounds when triggered by an incautious boot).
The robot keeps a very careful record of where these devices are placed, laying them randomly within a designated secret region. It then parks itself prominently somewhere where there is no mine.
The approaching army sees the bot, understands there is a minefield ahead and makes a cellphone call to the number displayed on its casing.
This causes the robot to start retracing its steps, neutralising the mines by firing them vertically upwards. This it does however exceptionally slowly.
When the process is complete, the bot destroys itself. The result is that a cheap minefield has delayed an oncoming army, been completely cleared and left no technology behind to be ‘repurposed’.