TAIPEI—Donald Trump took the call. The voice on the other end of the line was Taiwan’s president congratulating him. They chatted for a few minutes about economic matters and security—the normal business of politics. Why all the fuss?

After all, China didn’t object too strenuously, directing its displeasure primarily toward Taiwan for what it called a “petty trick.” That’s far from the explosion Beijing’s past behavior may have indicated for such a breach of protocol: No president-elect, or president, has spoken to a Taiwan...