On December 1, in Buenos Aires, President Trump started the 90-day clock to negotiate a trade deal with China. He claims he wants to tackle the big systemic concerns involving theft of American intellectual property, the forced transfer of technology from American firms, and the state-driven nature of the Chinese economy. For trade watchers, the time frame for such ambitions sounds absurd. But they are not entirely out of reach. If Trump makes up with scorned friends in order to take on a common adversary, he could get a meaningful deal.