Global economic growth has crept back after a deep recession, and as recently as early 2018 a coordinated international expansion was underway. But progress has shown cracks in recent months, with trade flows slumping and manufacturing indexes pulling back from Asia to Europe.

The Morgan Stanley economist Chetan Ahya estimates that if Mr. Trump’s trade war with China isn’t resolved and the administration follows through with its threats to increase tariffs, growth could fall enough that “we could wind up in a global recession in about three quarters.” Risks seem to have abated slightly after the recent Group of 20 meeting, where Mr. Trump suspended a tariff escalation and restarted trade talks with China.

But uncertainties persist. Those talks could crumble again, leading to additional import taxes. And beyond America’s trade wars, the threat of a disorderly British withdrawal from the European Union and a continuing slowdown in China pose further risks to international activity.

Those factors prompted Mr. Draghi to strongly signal in June that the central bank was planning to revive stimulus measures it had used during the eurozone debt crisis.

While Mr. Draghi insisted the bank still had “considerable headroom” to buy bonds as a way of pumping money into the economy, some analysts think he acted pre-emptively precisely because he knows the central bank’s capacity is finite. Better to use the bank’s limited resources now when they can still do some good.

In the United States, the Fed is also considering acting sooner rather than later as it tries to judge whether a rate cut is warranted. Emerging research suggests that moving quickly and decisively might be the central bank’s best defense.

While the Fed is in comparatively good shape because it has gotten rates off rock bottom — they’re at 2.25 to 2.5 percent — that leaves it just half as much room to cut borrowing costs as policymakers had back in 2007. In fact, the Fed’s chair, Jerome H. Powell, has started a yearlong review of just what its options are.