Thus far, the idea of supplying Ukraine through a spending bill has brought scorn from congressional Republicans who wonder whether Americans’ taxes wouldn’t be better spent on defending the U.S.-Mexico border. The REPO Act, by contrast, could make “Russia foot the bill for its own aggression,” as a group of Brookings Institution scholars puts it. Mr. Johnson himself calls it “pure poetry.” It is a tempting idea.

But it is a bad one. ...

If Russia, China and other diplomatic rivals were to decide that their dollar assets were vulnerable and that they could no longer trust the dollar as a means of exchange, we would feel the pain of that $34 trillion in debt in a way that we don’t now. Retaining the advantages of a reserve currency depends on our behaving as a trustworthy and neutral custodian of others’ assets. If we start stealing people’s money, that could change.