This newspaper's editorial columns a couple of months ago branded President Trump's tariffs on Canadian and Mexican imports "the dumbest trade war in history." Alas, my colleagues and I might have spoken too soon. A much, much dumber one is lurking just around the corner: an export tariff on U.S. Treasurys.

Welcome back to Intellectual Trumpism. If you gaze past Mr. Trump's personal bluster and inconsistency on trade policy, it's starting to look as if the White House is manifesting the views on the global economy of a circle of unorthodox economists. As Mr. Trump implements -- granted, haltingly -- these economists' ideas on tariffs, we should take seriously the risk that the administration will push ahead with some of their other ideas, this time on global financial markets.

The theory within Mr. Trump's circle is that the global economy is badly distorted by policy decisions in countries such as Japan, Germany and China to suppress domestic consumption and boost exports. Because the global economy is a closed system, these export surpluses necessarily create import surpluses elsewhere, and especially in the U.S.

Last week I focused on the physical side of this system, concerning the trade deficit. There's a flip side concerning capital flows. The Trumpist idea is that historical factors have pushed America into the role of furnishing the world's safe assets, particularly the dollar and the Treasury note. Global demand for these assets is enormous -- the world economy would judder to a halt without them -- and satisfying that demand forces the U.S. to run a trade deficit.

This argument comes in several forms. The broadest frets about all foreign investment in America. Sustained demand for dollars overseas causes the greenback to be perpetually overvalued, the idea goes. This overvaluation kneecaps exporters, inhibiting a natural rebalancing of the trade account. And the longer these imbalances persist (and the more debt the U.S. government, households and companies take on), the less stable the global financial system becomes.

Tariffs are one potential fix, insofar as they disrupt the trade flows that match these financial flows. The alternative solution is some sort of broad capital control. Happily, this idea would be so difficult to implement that it's unlikely the White House will try.

The real danger is that we get the narrower, more practical version: controls focused on the market for Treasurys. U.S. government securities "become exported products which fuel the global trade system," Stephen Miran, now chairman of Mr. Trump's Council of Economic Advisers, wrote in a widely circulated paper. "In exporting [those securities], America receives foreign currency, which is then spent, usually on imported goods. America runs large current account deficits not because it imports too much, but it imports too much because it must export [Treasurys] to provide reserve assets and facilitate global growth."

Mr. Miran helpfully summarized three potential "solutions" to this "problem." One is the "Mar-a-Lago Accord" you keep hearing about. This is shorthand for U.S.-coordinated global action by foreign governments to devalue the greenback and revalue other currencies. The inspiration for both the concept and the name is the Plaza and Louvre accords of 1985 and 1987, respectively, which arrested a rapid dollar appreciation. Mr. Miran's second idea is for the U.S. to accumulate its own foreign-exchange reserve of foreign governments' bonds to manage the dollar exchange rate.

The third proposal counts as the single worst idea ever floated by anyone associated with either Trump administration about anything: a tax on foreign holdings of Treasury securities.

To discourage the foreign reserve accumulation that supposedly drives the U.S. trade deficit, the thinking goes, Washington should discourage foreigners from purchasing dollar-denominated assets, perhaps by imposing a tax on foreign governments' holdings of Treasurys. Such a measure, which Mr. Miran dubbed a "user fee," would withhold some portion of the interest payments Treasury remits to foreign governments that own American bonds.

"Reserve holders impose a burden on the American export sector," Mr. Miran wrote last year of this proposal, arguing a user fee "can help recoup some of that cost." Mr. Miran said in the fall that his paper shouldn't be taken as policy advice. But he appeared to be alluding to the idea in a speech in his official capacity at the Hudson Institute this month, in which he suggested other governments "could simply write checks to Treasury" to share the burden that the U.S. supposedly bears as the provider of global reserves.

To understand why this is such a bad idea, it's worth digging into the premises of this argument. Is it true that trade deficits are foisted on unwilling Americans by the operation of foreign dollar hoarders and currency manipulators?

No. There's mixed evidence at best for a connection between foreign dollar-reserve accumulation and the U.S. trade deficit. Nor is it clear that foreign governments are accumulating Treasurys in the ways that Trumpists worry are trade-distorting. Even without a "user fee," foreign governments' Treasury holdings in reserve funds have declined to about 16% of the total float of U.S. government debt held outside the Federal Reserve, from a peak of about 40% immediately after the 2008 financial panic.

This undermines a key element of this Trumpist theory about the U.S. trade deficit: Clearly Washington doesn't have to keep issuing huge quantities of Treasury securities to satisfy global demand for safe assets. The rest of the world is finding plenty of other financial havens as is.

The real link between the dollar, foreign investors and the trade deficit is that America's status as issuer of the world's preferred safe assets means the rest of the world is happy to buy as much debt (government or private) and equity as we sell to finance our political choice to subsidize domestic consumption -- a political choice that is popular here in the U.S.

Or rather, the rest of the world is happy to buy that debt so long as we're not defaulting on it. And make no mistake, such a capital tax would be a default. That's what one calls it when a debtor unilaterally reneges on all or part of a promised repayment.

The great folly of a capital tax on Treasurys is that it would undermine the desirability of those assets even as our fiscal deficit continues to rage more or less out of control. I argued last week that Mr. Trump's trade protectionism is a roundabout way of avoiding politically painful entitlement reforms. Wait to see how excruciating such budgetary decisions will become if Washington faces a sustained global selloff of Treasurys.