First World War: The German submarine U-9 sank three Royal Navy cruisers, resulting in approximately 1,450 deaths.

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.

SM U-9 was a German Type U 9 U-boat. She was one of 329 submarines serving in the Imperial German Navy, and engaged in commerce raiding (Handelskrieg) during World War I.

The Action of 22 September 1914 was an attack by the German U-boat U-9 that took place during the First World War. Three obsolete Royal Navy cruisers of the 7th Cruiser Squadron manned mainly by Royal Naval Reserve part-timers and sometimes referred to as the Live Bait Squadron, were sunk by U-9 while patrolling the southern North Sea.

The Royal Navy (RN) is the naval warfare force of the United Kingdom. It is a component of His Majesty's Naval Service, and its officers hold their commissions from the King. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against France. The modern Royal Navy traces its origins to the English Navy of the early 16th century; the oldest of the UK's armed services, it is consequently known as the Senior Service.