The economists Milton Friedman and Anna Jacobson Schwartz demonstrated in “A Monetary History of the United States” that a collapse in the quantity of money was the main cause of the Great Depression. Hoping to avoid a repeat, the Federal Reserve in recent weeks has poured money into the economy at the fastest rate in the past 200 years. Unfortunately, this overreaction could turn out just as poorly; history suggests the U.S. will soon see an inflation boom.

Friedman and Schwartz used a broad definition of the quantity of...