Transatlantic alliance due for a reset. But what about China?

As tweets go, certainly by the standards of the outgoing Trump administration, it was downright benign, the cyber equivalent of a tap on the shoulder. Yet it heralds a key diplomatic challenge that awaits President-elect Joe Biden: forging a united response with America’s allies to the growing ambition and assertiveness of China.

The tweet came from Jake Sullivan, Mr. Biden’s choice for national security adviser. He was reacting to news that the European Union was poised, after nearly seven years of negotiations, to reach agreement in principle on a major...