World War I: U-boat campaign: Two days after the United Kingdom had declared war on Germany over the German invasion of Belgium, ten German U-boats leave their base in Heligoland to attack Royal Navy warships in the North Sea.

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 U-boat campaign from 1914 to 1918 was the World War I naval campaign fought by German U-boats against the trade routes of the Allies, largely in the seas around the British Isles and in the Mediterranean, as part of a mutual blockade between the German Empire and the United Kingdom.

U-boats are naval submarines operated by Germany, including during the First and Second World Wars. The term is an anglicized form of the German word U-Boot, a shortening of Unterseeboot. Austro-Hungarian Navy submarines were also known as U-boats.

Heligoland is a small archipelago in the North Sea. The islands were historically possessions of Denmark, then became possessions of the United Kingdom from 1807 to 1890. Since 1890, they have been part of the German state of Schleswig-Holstein, although after World War II they were managed by the United Kingdom as part of the British occupation zone in Germany from 1945 to 1952.