Answer:
Sinking of American merchant ships
Explanation:
Even before World War I had broken out, American opinion had been overall more negative toward Germany than toward any other country in Europe. Over time, especially after reports of atrocities in Belgium in 1914 and following the sinking of the passenger liner RMS Lusitania in 1915, American citizens increasingly came to see Germany as the aggressor in Europe.
In 1917, with Russia experiencing political upheaval, and with Britain and partly-occupied France low on credit, Germany appeared to have the upper hand in Europe, while the Ottoman Empire, Germany's ally, held on to its territory in modern-day Iraq, Syria and Palestine. However, a British economic embargo and naval blockade was by now causing shortages of fuel and food in Germany, at which point Germany decided to resume unrestricted submarine warfare. The aim was to break the transatlantic supply chain to Britain, although the German high command realized that sinking American-flagged ships would almost certainly bring the United States into the war.
