The president’s plan for reciprocal tariffs sounds good in theory. But there was a reason the United States abandoned the approach a century ago. The gains would be few and the costs enormous.

The president said his advisers would devise new tariff levels reflecting countries’ tariffs, taxes, subsidies and other policies affecting trade with the United States.

President Donald Trump is likely to dust off a 1930 trade law largely forgotten for decades to back his new reciprocal U.S. tariffs that will match other countries' higher import taxes, trade and legal experts say.