Trump promised to restore the heartland with tariffs on imported goods during his presidential campaign, often in offhanded and extreme terms and often targeting China in particular. “They want our people to starve,” he said before becoming president, saying there was “no choice” but to start a trade war with the country. (He added that he would serve McDonald’s to the Chinese leader at any future state dinner. Earlier this year, at an actual state dinner, he served dover sole with champagne sauce and dry-aged steaks.)

In April, Trump ordered the Department of Commerce to investigate steel imports under a little-known part of the Trade Expansion Act of 1962, Section 232, which allows the executive branch to place import restrictions or tariffs on steel for national security reasons. “Steel and aluminum are vital for U.S. national defense and critical infrastructure,” Robert E. Scott of the Economic Policy Institute, a left-of-center think tank, said in a piece supporting the idea. “The military needs high quality steel and aluminum to make products ranging from helmets and tanks to rocket fuel, fighter jets and aircraft carriers.” Foreign trade practices have made it impossible for the United States to supply its own steel for defense purposes, the theory goes.

But many trade experts, as well as foreign countries and American industry leaders, have raised questions about applying tariffs on those grounds. Trump has been vocal about wanting to punish China, rather than stressing that the state of the domestic steel industry is a national security problem. Trade experts argue that using Section 232 to achieve a non-security objective would be a misuse of a policy meant for true emergencies and a violation of norms and trade law. The case is “likely to raise barriers that don’t meet the international legal standards of the World Trade Organization, leading to disputes at the W.T.O. and retaliation from trading partners,” argues Chad P. Bown of the Peterson Institute of International Economics, a Washington-based think tank.

Thus, retaliation by exporters hit by the tariff (likely China) and importers that would absorb the glut of steel no longer headed to the United States (the European Union, among others) seems a certainty. Numerous other countries have voiced their opposition to the potential Trump move for months. The Chinese government “believes there is no evidence that steel imports threaten to impair U.S. national security,” Yu Gu, a Chinese official, testified at a Commerce Department hearing earlier this year. “The United States’ defense and national security requirements are plainly not dependent on imports of foreign-made steel.” The E.U. has sworn to fight back if the Trump White House goes ahead and is reportedly already preparing measures, such as levies on American whiskey, rum, orange juice, potatoes, and tomatoes.