During World War I, Germany did an impressive job turning sympathy into hostility. At the start of the war the Royal Navy blockaded German ports. The blockade was also extended to neutral ports like those in the Netherlands, under the logical argument that they were being used to import materiel that kept the German war effort going. It was an unpopular argument with neutrals like the United States. The hardship and starvation inflicted on German civilians as a result of the blockade gave rise to much sympathy for the German cause in the United States – aided by the large population of German descent.
Meanwhile the German admiralty was reluctant to risk the shiny new navy that had turned the United Kingdom into a rival to the vagaries of an open sea battle – it must be noted that the Royal Navy keenly aware that Britain’s survival depended on control of the sea had no desire to force the issue either. Having to justify their existence the German Navy turned to submarine warfare. If Britain would strangle Germany by blockade, Germany would return the favor by sinking the British merchant fleet.
The problem was that unlike the British blockade the U Boat campaign would attack ships flying neutral flags and passenger ships – i.e. directly kill civilians. On May 7, 1915 came the most infamous such strike. A German submarine sank the Cunard liner RMS Lusitania. 1,198 of the 1,959 people aboard were killed (including 129 of the 138 Americans on board). It was manna from heaven for the British propaganda machine aided by the unrepentant German response that tried to justify the sinking.
The sinking of the Lusitania was a giant blow to support for Germany in World War I – made worse by the later unrestricted U-Boat campaign – and public opinion gradually turned anti-German. In 1917 came the compounding idiocy of the Zimmerman Telegram ( a diplomatic proposal by Germany to Mexico to attack the United States that was intercepted by the British) that finally tipped the Americans into war with Germany.