Smoot-Hawley Tariff Act

Or FDR's Hoot-Smalley.

June 17, 1930:

U.S. President Herbert Hoover signs the Smoot-Hawley Tariff Act into law.
The Tariff Act of 1930 (codified at 19 U.S.C. ch.4 ), otherwise known as the Smoot–Hawley Tariff or Hawley–Smoot Tariff, was an act, sponsored by Senator Reed Smoot and Representative Willis C. Hawley, and signed into law on June 17, 1930, that raised U.S. tariffs on over 20,000 imported goods to record levels.
The overall level tariffs under the Tariff were the second-highest in U.S. history, exceeded by a small margin only by the Tariff of 1828. The act, and the ensuing retaliatory tariffs by U.S. trading partners, reduced American exports and imports by more than half.
At first, the tariff seemed to be a success. According to historian Robert Sobel, "Factory payrolls, construction contracts, and industrial production all increased sharply." However, larger economic problems loomed in the guise of weak banks. When the Creditanstalt of Austria failed in 1931, the global deficiencies of the Smoot–Hawley Tariff became apparent.
U.S. imports decreased 66% from US$4.4 billion (1929) to US$1.5 billion (1933), and exports decreased 61% from US$5.4 billion to US$2.1 billion, both decreases much more than the 50% decrease of the GDP. Thus exports minus imports which is the GDP formula declined from 1 billion to 600 million while GDP was 58.9 billion, a trivial effect on GDP of about 2/3 of 1%.
According to government statistics, U.S. imports from Europe decreased from a 1929 high of $1,334 million to just $390 million during 1932, while U.S. exports to Europe decreased from $2,341 million in 1929 to $784 million in 1932. Overall, world trade decreased by some 66% between 1929 and 1934.

A global trade war didn't help end the Great Depression.  So far as I know, this is one of the mistakes we have avoided making in the Great Recession.  Unfortunately, we've been making a number of others.