Trump advisors argue steep tariffs on countries like Vietnam and Cambodia are needed to erase trade deficits and rework supply chains. The economics say that’s going to be a long shot

U.S. President Donald Trump, on “Liberation Day,” reserved some of his highest tariff rates for Southeast Asia. Economists expected to see some targets, like manufacturing hub Vietnam, on Trump’s tariffs list. Other targets, like neighboring Cambodia, were more surprising—and all were shocked by the steep tariffs imposed on the region, often extending into the 40% range. 

April 2 set off a scramble in Southeast Asia, which has relied on exports for growth. Both Vietnam and Cambodia have...