We may be a violent species, but some taboos do, clearly, have some valuable restraining effect. One of those, the notion that nuclear weapons should only be used as a weapon of last resort, is clearly eroding both among terrorist groups and an increasing number of nations. That said, it’s still difficult to read about the administration’s plans to be allowed to research so-called “mini-nukes” (five kilotons or less) without profound unease. This is not a precedent that the US needs to be setting.