Last month, before an audience of world leaders, Prime Minister Shinzo Abe of Japan forcefully defended the global trade order that President Trump has so dramatically fractured.

“A free and open economy,” he told leaders of the Group of 20 nations in Osaka, Japan, “is the foundation of global peace and prosperity.”

Two days later , Mr. Abe became the latest world leader to strike a blow against free trade, when he moved to limit South Korea’s access to Japanese chemicals that are essential to its vast electronics industry, citing vague and unspecified concerns about national security. In doing so, Japan joined the United States, Russia and other countries that have used national security concerns as a justification for cutting off trade.

Once rarely invoked by world leaders, such arguments are wearing away at long-established global rules intended to keep trade disputes from spiraling out of control. Once they are weakened, experts say, damaging trade wars could become more common.