President Trump has named his foreign policy “America First” after the failed isolationist policies promoted by the Republican Party in the 1930s. But he often seems to act more in accord with another unsuccessful Republican approach to international affairs: the “dollar diplomacy” practiced between 1909 and 1912 by President William Howard Taft and his secretary of state, Philander Knox, a corporate lawyer who helped form U.S. Steel.

Claiming they were substituting “dollars for bullets,” the portly Taft and the peppery Knox subordinated U.S. foreign policy to the interests of U.S. corporations. They promoted loans from Wall Street banks to developing countries from the Caribbean to China and sent in the Marines when U.S. property was at risk from revolutionaries. There was at least a patina of idealism to this commerce-first policy, because Taft and Knox assumed — wrongly, as it turned out — that U.S. investments could stabilize war-torn lands.

Trump’s dollar diplomacy, by contrast, is devoid of any hint of higher purpose. His is the most amoral U.S. foreign policy ever. It is all about “winning” for the United States, which he defines in exactly the same dollars-and-cents way that he did as a property developer. It is as if he were the chief executive of United States Inc., and his only purpose were to deliver a profit for its 325 million stockholders. If he is familiar with the creed upon which this country was founded — “life, liberty and the pursuit of happiness” — he gives no sign of it. All he cares about is the pursuit of lucre.

That Trump sees the world almost exclusively through a commercial lens helps to explain what he gets exercised about — and what he doesn’t. He gets angry about Canadian tariffs on U.S. milk exports — a “disgrace” and “not fair to our farmers.” He held U.S.-Canada relations hostage for more than a year, causing U.S. standing in our northern neighbor to plummet and potentially damaging one of the world’s closest alliances, just so he could win a minuscule advantage for U.S. dairies.

Trump is also angry about the trade surplus that nations such as Germany, Japan and China run with the United States, because he doesn’t understand what it means. “We lost, over the last number of years, $800 billion a year,” he said in March. “Not a half a million dollars, not 12 cents. We lost $800 billion a year on trade.” Of course, the U.S. doesn’t “lose” on trade: We got a lot of stuff back, from sneakers to computers. But Trump is willing to jeopardize not only our trading relationships but also strategic alliances in his misguided effort to reduce the U.S. trade deficit. (The trade deficit has actually gone up on his watch, because the economy is booming and consumers can afford to buy more foreign goods.) ...