In a speech last week, Janet L. Yellen, the Treasury secretary, said the pandemic and the war had revealed that American supply chains, while efficient, were neither secure nor resilient. While cautioning against “a fully protectionist direction,” she said the United States should work to reorient its trade relationships toward a large group of “trusted partners,” even if it meant somewhat higher costs for businesses and consumers.

Ngozi Okonjo-Iweala, the director general of the World Trade Organization, said in a speech on Wednesday that the war had “justifiably” added to questions about economic interdependence. But she urged countries not to draw the wrong conclusions about the global trading system, saying it had helped drive global growth and provided countries with important goods even during the pandemic.

“While it is true that global supply chains can be prone to disruptions, trade is also a source of resilience,” she said.

The W.T.O. has argued against export bans since the early days of the pandemic, when countries including the United States began throwing up restrictions on exporting masks and medical goods and removed them only gradually.

Now, the Russian invasion of Ukraine has triggered a similar wave of bans focused on food. “It’s like déjà vu all over again,” Mr. Evenett said.

Protectionist measures have cascaded from country to country in a manner that is particularly evident when it comes to wheat. Russia and Ukraine export more than a quarter of the world’s wheat, feeding billions of people in the form of bread, pasta and packaged foods.

Mr. Evenett said the current wave of trade barriers on wheat had begun as the war’s protagonists, Russia and Belarus, clamped down on exports. The countries that lie along a major trading route for Ukrainian wheat, including Moldova, Serbia and Hungary, then began restricting their wheat exports. Finally, major importers with food security concerns, like Lebanon, Algeria and Egypt, put their own bans into effect.