More: The end of U.S. economic leadership. Britain played this role through World War I, but it was too weakened by war to continue. The U.S. didn't take up the leadership mantle until after depression and World War II. U.S. leadership and the decision to spread free trade produced seven decades of mostly rising prosperity at home and abroad. The U.S. share of global GDP has been stable at about 25% for decades, even as industries rise and fall.

That era is now ending, as Mr. Trump adopts a more mercantile vision of trade and U.S. self-interest. The result is likely to be every nation for itself, as countries seek to carve up global markets based not on market efficiency but for political advantage. In the worst case, the world trading system could devolve into beggar-thy-neighbor policies as in the 1930s.

The cost in lost American influence will be considerable. Mr. Trump thinks the lure of the U.S. market and American military power are enough to bend countries to his will. But soft power also matters, and that includes being able to trust America's word as a reliable ally and trading partner. Mr. Trump is shattering that trust as he punishes allies and blows up the USMCA that he negotiated in his first term.

A major opportunity for China. The great irony of Mr. Trump's tariffs is that he justifies them in part as a diplomatic tool against China. Yet in his first term Mr. Trump abandoned the Asia-Pacific trade deal that excluded China. Beijing has since struck its own deal with many of those countries.

Mr. Trump's new tariff onslaught is giving China another opening to use its large market to court American allies. South Korea and Japan are the first targets, but Europe is on China's list. Closer trade ties with China, amid doubts about access to the U.S. market, will make these countries less likely to join the U.S. to impose export controls on technology to China or to ban the next Huawei.