Phil Gramm and Donald J. Boudreaux writes:

Equities markets roared in anticipation of the deregulation, tax cuts and fiscal restraint the second Trump administration promised. But the tariff war with Mexico and Canada has destroyed that optimism and wiped out the increase in the value of American equities that had accrued since November. On April 2, President Trump plans to take his protectionist policies global. If he does, it will be the beginning of the end of the trading system that built the modern world.

Mr. Trump’s policy turns the traditional meaning of reciprocal trade on its head. He wants to achieve reciprocity only by raising tariffs, almost certainly triggering retaliation.

Reciprocal trade policy as envisioned by President William McKinley, whom Mr. Trump often cites as his role model, recognized that by the dawn of the 20th century America had emerged as an economic colossus capable of producing an abundance of products that could be profitably exported. As McKinley explained, “the expansion of our trade and commerce is the pressing problem. Commercial wars are unprofitable.”

McKinley’s reciprocal trade policy was aimed at opening markets for U.S. products with agreements that lowered tariffs on imported products proportionately as other countries lowered theirs on U.S. products. President Franklin D. Roosevelt used reciprocal trade policies to back the world out of the Smoot-Hawley tariff. His successors used reciprocal trade to lift the majority of the world’s population out of poverty and achieve 75 years of peace and prosperity.

The Trump perversion of reciprocal trade co-opts a politically appealing phrase to justify his preferred policy. While the president uses European and Japanese tariffs on American cars to justify comparable tariffs on U.S. imports, nowhere does he propose real reciprocity.

He could eliminate the 25% U.S. tariff on imported trucks as an inducement to other countries to eliminate their tariffs on U.S. automobiles. Mr. Trump denounces high tariffs on U.S. exports to Central and South America and would use his theory of reciprocity as an excuse to raise U.S. tariffs on imports from those countries. But real reciprocity would be achieved by eliminating the quota on U.S. imports of sugar, for which Americans pay twice the world price, in return for Central and South American countries lowering their tariffs against U.S. products.

The timing of Mr. Trump’s actions could hardly be worse. America now dominates the tech industries. Because of our strong comparative advantage in high-tech and artificial intelligence, these industries are obvious targets for retaliation.

There is something to the claim that history repeats itself. The trade wars that wrought havoc in the 20th century occurred because of America’s efforts to protect agriculture from foreign competition in the 1920s and ’30s, when we had seized a strong comparative advantage in industrial production. World War I had crippled European agriculture as millions were pulled off farms to fight. Crop prices soared and American farmers enjoyed an economic heyday. But when the war ended, agriculture production recovered in Europe and world farm product prices plummeted.

Because the farm vote was the marginal vote in national elections, Congress rushed to pass the 1922 Fordney-McCumber tariff to protect American agriculture. When that tariff failed to alleviate the agriculture distress, Herbert Hoover, running for president in 1928, promised to support additional farm tariff protection. Efforts to provide that protection ultimately produced the Smoot-Hawley Tariff Act of 1930 as industry piggybacked on the support for farm tariffs to pile on industrial tariffs as well.

The result helped turn a financial panic into a worldwide depression.

Tragically, the 1922 tariff made it virtually impossible for Germany to pay its war debts, triggering devastating hyperinflation. We all know what happened next: Smoot-Hawley helped usher in a global depression with its economic and political carnage.

Politics in America sped the collapse of the world market in an effort to protect agriculture when it was a declining source of employment and income. At the same time, this protectionism denied a world market to U.S. manufacturing, where employment and wages were rising.

Why was the 2024 election so focused on manufacturing jobs? The percentage of jobs in manufacturing has been in secular decline for more than three quarters of a century. Wages in manufacturing are lower on average than wages in the service industries. Technology has continued to expand America’s industrial capacity while employment in manufacturing as a share of total nonfarm employment has fallen by 75% since 1946.

Manufacturing jobs were at the center of the 2024 election because industrial workers have become swing voters. We are today taking actions to protect manufacturing jobs the same way we did with agriculture a century ago. In the process, we are imperiling our access to the world market in high-tech and AI, which are the economic future.

There is no lesson in the second kick of a mule, but it is important to remember that under the protectionist policies of the first Trump administration the trade deficit rose, employment in manufacturing as a percentage of total employment continued to decline, and economic growth, which reached a 13 year high in 2018 under Mr. Trump’s deregulatory and tax-cut policies, slumped under his tariffs. This was all before the pandemic started.

Protectionism has an unblemished record of failure both economically and politically throughout American history. Yet the Trump administration seems determined to employ protectionist policies that failed the first time it employed them and that have never spurred economic growth. Protectionism now threatens not only the prosperity that could be created with deregulation, budget-deficit reductions and tax cuts, but also imperils America’s world leadership and the peace and prosperity that leadership has produced.