Everyone said Old Europe was dying. Sure doesn’t look like it now.

The contrast could not be more stark. Theresa May, the British prime minister, presides over a hung Parliament and a divided country. Donald Trump, the American president, rules alongside a Congress almost too paralyzed to legislate. In both countries, far-left and far-right movements and ideas have more adherents than ever; political debate is angry, hate-filled — and violent. Gunmen have now shot at U.S. lawmakers on the left and right; in Britain last year, an MP was murdered.