I saw on an AHC program the German calculation about unrestricted sub warfare: that they could choke Britain in six months, but it would take the US two years to ramp up a real fighting force. So it made good sense, but a miscalculation in a different way.
It was a severe miscalculation. First, Germany already had a strategy to win the war - beat Russia first - and any attempt to knock out Britain at the cost of involving the US was a major distraction. Second, even if US troops took longer to arrive, the US Navy was ready to help defeat the submarine campaign right away [1].
