As American steelmakers announce huge yearly profits, President Trump has practically unfurled a “Mission Accomplished” banner. “Tariffs on the ‘dumping’ of Steel in the United States have totally revived our Steel Industry,” he tweeted last week. Yet investors don’t think so. From the time Mr. Trump announced his 25% tariffs through the middle of January, steel stocks lagged the S&P 500 by nearly 20 percentage points. What gives?

One possible explanation is that steel companies have reaped a one-time windfall. Tariffs caused steel prices to spike last year, hence all the superlatives on the recent investor calls.

• “The best way for me to sum up 2018 is this: It was a record year for Nucor, ” said CEO John Ferriola. Nucor’s profit was up $1 billion, or 79%, compared with the previous year.

• “On many measures, 2018 was a record year—record operating metrics, record profitability and record cash generation, to name just a few,” said Mark Millett, CEO of Steel Dynamics. Profit increased $445 million, or 55%.

• “Our best year in a decade,” said Roger Newport, CEO of AK Steel, where profit rose $83 million, or 80%.

• “We delivered our best return on capital employed since 2008,” said David Burritt, CEO of U.S. Steel. His profit? Up $728 million, or 188%.

The common theme: higher prices, strong sales, and a sharp drop in foreign competition. “Increased demand levels and lower imports generated approximately six million tons of added volume for the U.S. steel industry last year,” Mr. Ferriola said. Mr. Millett said imports of finished steel declined “on both an absolute basis and as a percentage of consumption.”

So while prices surged, to the detriment of America’s consumers and steel-reliant industries, a handful of mills were minting money. Yet the fundamentals of the global steel market haven’t changed, and steel prices have dropped back toward the baseline. Estimates suggest 2019 will be nowhere near as good.

And what happens if Mr. Trump strikes a bargain to lift the tariffs? As he pushes Canada and Mexico to pass his new Nafta deal, they may first insist, understandably, on relief for their steel producers. The U.S. still lacks enough domestic steelmaking capacity to meet demand and so imports are essential to American manufacturing.

Yet Mr. Trump’s promise to bring back steel jobs for the workingman simply hasn’t happened. In November there were 83,400 people employed by American iron and steel mills, according to data from the Labor Department. That’s down 100 from last February, the month before Mr. Trump’s tariff announcement.

In 2008, the last time steel companies were busting financial records, the employment figure was touching 100,000. It isn’t a failure that steelmakers have made productivity gains, even if Mr. Trump gives a campaign speech in Pittsburgh and tries to present it that way.

Newsletter Sign-up Opinion: Morning Editorial Report All the day's Opinion headlines. PREVIEW

Meanwhile, Mr. Trump’s protectionism has hammered businesses that depend on steel. Ford said last month that tariffs cost the car maker $750 million last year, along with $1.1 billion in higher spending on steel and aluminum. As a result, the profit-sharing checks going to Ford’s hourly workers were reduced by somewhere around $750 to $1,850 each, according to the Detroit Free Press.

Caterpillar’s costs tied to tariffs were $100 million, and the company says they will double in 2019. Whirlpool, a supposed beneficiary of Mr. Trump’s protectionism, expects its bill next year for tariffs and pricier raw materials will hit $300 million.

To calculate the total tariff damage, multiply this effect across every steel buyer nationwide, public and private: metal fabricators, nail manufacturers, pipeline builders, keg makers, and so forth. Then add in the harm that retaliatory tariffs from U.S. trading partners have done to America’s soybean farmers, cranberry growers, bourbon distillers, pork producers, lobster trappers, and so many more. Then add the harm to investment from political uncertainty.

Tariffs offer the classic political problem of concentrated benefits and diffuse losses. Steel executives get to strut with their record profits, earned under Uncle Sam’s protection. Mr. Trump gets to proclaim himself a blue-collar savior, despite flat steel employment. Everyone else picks up the tab.