World War I: Adhering to the terms of the Treaty of London, the United Kingdom declared war on Germany in response to the latter's invasion of Belgium.

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 Treaty of London of 1839, was signed on 19 April 1839 between the major European powers, the United Kingdom of the Netherlands, and the Kingdom of Belgium. It was a direct follow-up to the 1831 Treaty of the XVIII Articles, which the Netherlands had refused to sign, and the result of negotiations at the London Conference of 1838–1839 which sought to maintain the Concert of Europe.

The German invasion of Belgium was a military campaign which began on 4 August 1914. On 24 July, the Belgian government had announced that if war came it would uphold its neutrality. The Belgian government mobilised its armed forces on 31 July and a state of heightened alert was proclaimed in Germany. On 2 August, the German government sent an ultimatum to Belgium, demanding passage through the country and German forces invaded Luxembourg. Two days later, the Belgian government refused the German demands and the British government guaranteed military support to Belgium. The German government declared war on Belgium on 4 August; German troops crossed the border and began the Battle of Liège.