The outcry for higher tariffs could really intensify if the economy takes a serious turn for the worse, as it did after the stock market crash of 1929. Then, factories shut down and unemployment spiked. That led to the passage of Smoot-Hawley, which triggered a sharp reaction from foreign governments. Global trade slowed sharply and did not fully recover until the 1970s.

Today, however, the world economy looks much more like it did in the 1920s than in the depressed ’30s. Unemployment and inflation are low, growth is reasonably strong, inequality is back to levels last hit in the 1920s, and the mood is shifting against globalization. Still, the clamor for protection could grow significantly in intensity.

The obvious concern: If Mr. Trump is pushing tariffs during good times, just imagine what he might do if the times turn bad. That’s when governments normally become even more protectionist, and this administration is already moving in that direction. The decision by Mr. Cohn, the head the National Economic Council and a staunch defender of free trade, to leave the administration is clear evidence of the trend.

The next chapter in this period of deglobalization may revolve around how Mr. Trump deals with China. If the world’s two largest economies fall into a spiral of tit-for-tat retaliation, the global trend toward trade protection could pick up pace. China is a minor supplier of steel and aluminum to the United States but a major supplier of much else.

In the global financial markets, many observers are thus less concerned about the steel tariffs than about the Section 301 investigation launched by the Trump administration against China, which goes well beyond earlier investigations into illegal subsidies for export manufacturers and other stealth trade protection. Instead, the 301 investigation — named after the section of the Trade Act of 1974 that gives the president great leeway to address trade issues — is a broad look at Chinese practices such as investing in American companies to steal technology, penetrating American data networks and lifting technology from American companies that invest in China.

In short, the current controversy over steel and aluminum tariffs involves old industries that are shrinking as a share of the global economy and affect about 2 percent of global trade. Even if Mr. Trump’s suggested new tariffs provoke Chinese retaliation in these industries, it would have a manageable impact. A broader trade conflict involving newer sectors such as technology, or that leads to restrictions on foreign investment, could be much more damaging.

Of course, this is not the first time in the postwar era that an American president has resorted to trade protectionism. Richard Nixon fought his 1968 election campaign on a mercantilist platform. Both Ronald Reagan and George W. Bush imposed trade tariffs. The difference then was that America exercised more dominance over the global economy and had greater reason to believe it could squeeze concessions from trading partners. Rising powers such as China are not likely to be as compliant now, and even allies in Europe seem to feel less obliged to placate America.

Regardless of how far Mr. Trump goes in pressing tariffs against both rivals and allies, it is important to remember that this is not the beginning of the story, or the end. We are somewhere in the middle. The seeds of discontent against globalization were sown following a period of intensive trade and migration that culminated in the financial crisis of 2008. The age of deglobalization is now a worldwide phenomenon that is larger than Mr. Trump. It was coming whether he won election or not. And it is unlikely to end until long after he is gone.