Which u. s policy decreased import the united states in the 1920s making it difficult for european nation to repay their war debts

Protectionism in the form of tariffs on imported goods.  Tariffs were seen as a way of protecting and promoting American domestic products, as well as a way to gain government revenue.  But they hampered trade and thus impacted European nations' ability to repay war debts.

According to the Office of the Historian of the US State Department, the Fordney-McCumber Tariff Act of 1922 had exactly such an unintended impact in regard to repayment of war debts.  The aim of the tariff act was to provide protection for American farmers and industries, but in the end such tariffs hurt more than they helped.

Incidentally, on the subject of repayment of war debts from World War I, do you know the year in which Germany finally finished paying its war reparations to the Allies from that war?  It was 2010, nearly 100 years after the war had happened.  See the article, "How War Debts, High Tariffs, and Competitive Devaluation Led to War," by David Woolner in Business Insider, October, 2010. 

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