German U-boat calculations ignored interactions and misjudged British resolve

Evidence

In deciding whether to adopt unrestricted submarine warfare, the Germans estimated the amount of goods that Britain needed to maintain her position and the numbers of ships that the Germans thought they could sink. The conclusion was that they could quickly reduce the flow of material coming into Britain to below the minimum level; thus, Britain would sue for peace before the impact of U.S. entry into the war could be felt. While the specific calculations were accurate, the influence of one of the factors on the others was neglected. That is, once the United States entered the war, the British were willing to suffer what earlier would have been an intolerable loss of shipping because they realized that if they held on a bit longer the tide would turn (Ikle, 1971). — John T. Jost and Jim Sidanius, Political Psychology, 2588