In the chronicles of President Trump’s revolutionary trade policy, not enough attention has been paid to the destruction of a major villain. Ding-dong, de minimis is dead.

To understand how we got here, and why this change matters, you have to trace how the once obscure de minimis provision evolved from a Depression-era workaround to a 21st-century liability. For decades, the loophole, which eliminated tariffs on cheap imports, allowed overseas shippers to flood the U.S. market with small-value packages duty-free and without adequate inspection. In an executive order last month, Mr. Trump suspended the exemption for all countries and with one stroke reasserted the basic principle that all goods must face the full scrutiny, and the full weight, of U.S. law.

The move infuriated de minimis defenders. They claim that the carve out for small parcels allowed Americans to stretch their buying dollars while also benefiting small businesses.

But the new policy, which is to take effect on Aug. 29, is not about eliminating small packages. It is about resurrecting basic rules.

De minimis dates back to 1938, when Congress, citing expense and inconvenience, empowered customs officials to waive duties and fees on shipments worth less than $1, or about $23 today. It wasn’t worth the printing, stamping, stapling and filing necessary to assess tariffs on trifles, the thinking went.

Over time, though, de minimis distended. In 1978, rampant inflation prompted Congress to raise the de minimis ceiling to $5. By 1993, amid a global trend of waning tariffs, Congress sought to encourage trade liberalization by jacking up the limit to $200. Then, in 2015, legislators raised it to $800. (China, in comparison, has a de minimis threshold of less than $7.)