The U.S. is quietly — and largely with a view to a potential war against China — tweaking its nuclear strategy, contemplating a bigger role for so-called tactical nukes. To the uninitiated, the shift may seem arcane. To those in the know, the stakes are nothing less than the risk of atomic Armageddon. Obscure or not, the topic is so existential that it should top, or at least make, the list when U.S. President Donald Trump hosts his Chinese counterpart, Xi Jinping, later this month. If they don’t discuss it, after already avoiding their looming nuclear arms race when they talked in May, they will endanger the whole world. The adjustment in American nuclear strategy has been communicated in various speeches and comments by administration officials such as Elbridge Colby, a policy thinker in the Pentagon. It’s generally couched in phrases such as giving the president more “options,” with weapons that are “credible” because, as Colby puts it, a potential enemy has “got to believe there’s some sense in which we would use” them. That language is code for tactical (also