World War I: A meeting of the German Crown Council decided upon the resumption of unrestricted submarine warfare.

World War I or the First World War, also known as the Great War, was a global conflict between two coalitions: the Allies and the Central Powers. Main areas of conflict included Europe and the Middle East, as well as parts of Africa and the Asia-Pacific. There were important developments in weaponry including tanks, aircraft, artillery, machine guns, and chemical weapons. One of the deadliest conflicts in history, it resulted in an estimated 30 million military casualties, plus another 8 million civilian deaths from war-related causes and genocide. The movement of large numbers of people was a major factor in the deadly Spanish flu pandemic.

The 9 January 1917 Crown Council meeting, presided over by German Emperor Wilhelm II, decided on the resumption of unrestricted submarine warfare by the Imperial German Navy during the First World War. The policy had been proposed by the German military in 1916 but was opposed by the civilian government under Chancellor Theobald von Bethmann Hollweg who feared it would alienate neutral powers, including the United States.

Unrestricted submarine warfare is a type of naval warfare in which submarines sink merchant ships such as freighters and tankers without warning. The use of unrestricted submarine warfare has had significant impacts on international relations in regard to both the First World War and the Second World War. Its history has been dominated by German decision making.