Donald Trump’s plan to impose tariffs on Mexican exports unless our neighbor does something — he hasn’t specified what — to stop the flow of asylum-seekers is almost surely illegal: U.S. trade law gives presidents discretion to impose tariffs for a number of reasons, but curbing immigration isn’t one of them.

It’s also a clear violation of U.S. international agreements. And it will reduce the living standards of most Americans, destroy many jobs in U.S. manufacturing, and hurt farmers.

But let’s put all of that to one side and talk about the really bad stuff.

Trump says that “TARIFF is a beautiful word indeed,” but the actual history of U.S. tariffs isn’t pretty — and not just because tariffs, whatever the tweeter in chief says, are in practice taxes on Americans, not foreigners. In fact, it’s now a good bet that Trump’s tariffs will more than wipe out whatever breaks middle-class Americans got from the 2017 tax cut.

The more important fact is that until the 1930s, tariff policy was a cesspool of corruption and special-interest politics. One of the main purposes of the 1934 Reciprocal Trade Agreements Act, which eventually became the template for the modern world trading system, was to drain that particular swamp by removing the capriciousness of previous tariff policy.