Foreign companies and their workers were flattened by Smoot-Hawley’s steep tariff rates and foreign governments soon retaliated with trade barriers of their own. With their ability to sell in the American market severely hampered, they curtailed their purchases of American goods. American agriculture was particularly hard hit. With a stroke of the presidential pen, farmers in this country lost nearly a third of their markets. Farm prices plummeted and tens of thousands of farmers went bankrupt. A bushel of wheat that sold for $1.00 in 1929 was selling for a mere 30 cents by 1932.

With the collapse of agriculture, rural banks failed in record numbers, dragging down hundreds of thousands of their customers. Nine thousand banks closed their doors in the United States between 1930 and 1933. The stock market, which had regained much of the ground it had lost since the previous October, tumbled 20 points on the day Hoover signed Smoot-Hawley into law and fell almost without respite for the next two years. (The market’s high, as measured by the Dow Jones Industrial Average, was set on September 3, 1929, at 381. It hit its 1929 low of 198 on November 13, then rebounded to 294 by April 1930. It declined again as the tariff bill made its way toward Hoover’s desk in June and did not bottom out until it reached a mere 41 two years later. It would be a quarter-century before the Dow would climb to 381 again.)

The shrinkage in world trade brought on by the tariff wars helped set the stage for World War II a few years later. In 1929, the rest of the world owed American citizens $30 billion. Germany’s Weimar Republic was struggling to pay the enormous reparations bill imposed by the disastrous Treaty of Versailles. When tariffs made it nearly impossible for foreign businessmen to sell their goods in American markets, the burden of their debts became massively heavier and emboldened demagogues like Adolf Hitler. "When goods don’t cross frontiers, armies will," warns an old but painfully true maxim.