What if the next war goes nuclear because one side mistakes a conventional missile for a nuclear one, and responds accordingly? This issue of “entanglement,” or the risks of confusing conventional weapons with nuclear ones, and vice versa, deeply concerns us. It is also finally starting to receive the public policy attention it deserves: For example, whether to develop a nuclear variant of the U.S. Tomahawk sea-launched cruise missile has become a relatively important topic in the U.S. Congress.