Rattled by Trump, America’s Allies Shift to Defense Mode

President Trump’s unexpected plan to impose a 25 percent tariff on cars and car parts being imported into the United States will not only disrupt supply chains. It will also fuel anger, alienation — and pressure to retaliate — among American allies across the globe.

Many of the countries most affected by the new levies, such as South Korea, Japan, Germany, Mexico and Canada, are already reeling from the Trump team’s wanton disregard for free trade deals already signed, and erratic threats to long-established security relationships.

Prime Minister Mark Carney of Canada said on Wednesday that Trump’s move was “a direct attack.” Other leaders reacted in muted terms, hinting that they were still considering how to respond, with another round of tariffs expected in early April.

“We need to consider what’s best for Japan’s national interest,” Prime Minister Shigeru Ishiba of Japan told Parliament on Thursday. “We’re putting all options on the table in considering the most effective response.”

The tariffs, which threaten both American and foreign carmakers, increase the likelihood of a global trade war. A chain reaction of economic nationalism with tariffs and other measures — perhaps adding costs for finance and services — could suppress economic growth globally, spread inflation, and add rancor to already testy negotiations with Washington about security.

The Trump White House has made clear that it will use every tool of American power, including its military support and consumer market, to extract what Mr. Trump sees as a better deal for Americans. Economists argue that the potential benefit of more auto factory jobs could take years to show up in the United States, and they may coincide with losses in other industries. But for countries that have spent decades trusting America and tying their economies and defense plans to Washington’s promises, this feels like a moment of reversal.