LONDON — In ordinary times, worries about the health of the global economy tend to prompt leaders of the largest countries to join forces in pursuit of safety.

These are not ordinary times.

The biggest threat to global fortunes has become the intensifying conflict between the two largest economies on earth, the United States and China. As their leaders openly contemplate how to inflict pain on each other, the rest of the world now frets about becoming collateral damage in an escalating trade war.

Only a week ago, China and the United States appeared to be moving toward cooling their hostilities, while global economic prospects were improving. Worries about a worldwide slowdown were giving way to burnished hopes for expansion.

Fears about the weakening of China’s economy were easing as President Trump advertised a soon-to-be-signed trade deal. That lifted the outlook for Asian economies dependent on global commerce like Japan, South Korea and Taiwan. Europe, a perpetual source of concern, was flashing signs of renewal. Defying skeptics, the American economy remained on a tear.