RELIABILITY IS FOR losers. Strength is what keeps superpowers safe. That bleak philosophy, or some version of it, appears to unite the supreme leaders of China and America. After some weeks of fragile calm, their trade war is roaring back to life. Officials answering to Xi Jinping and Donald Trump keep exchanging threats and counter-threats. With each tit-for-tat round, the two giants give other trade partners fresh reasons to fear dependence on them. Neither seems greatly to care.