Britain Declares War on Germany
An ultimatum over Belgian neutrality expires and Britain enters a war that will kill nearly a million of its soldiers
Quick facts
- Ultimatum issued
- 2pm, 4 August 1914
- Deadline expired
- 11pm, 4 August 1914
- Immediate cause
- German invasion of neutral Belgium
- Daily enlistment by late August
- c. 30,000 men
What happened
Germany's invasion of neutral Belgium in early August 1914, combined with British fears of German domination of Europe, brought Britain and its empire into the First World War. At 2pm on 4 August 1914 Britain issued an ultimatum demanding Germany withdraw its troops from Belgium; when the deadline passed at 11pm without a German response, Britain was at war. The declaration was, by most accounts, greeted with popular enthusiasm and a rush to enlist, with around 30,000 men signing up daily by the end of August. Britain went on to declare war against Austria-Hungary eight days later and against the Ottoman Empire three months after that.
Why it matters
Britain's entry pulled its entire empire into a war that would kill roughly 900,000 British and Commonwealth soldiers and reshape the country's economy, politics, and society for a generation, accelerating the extension of the vote, the entry of women into industrial labor, and the eventual collapse of the pre-war European order.
How we know
The declaration of war and its immediate diplomatic run-up are documented in official Foreign Office correspondence and Cabinet records from early August 1914, held by the National Archives and the Imperial War Museums.
Sources
- Imperial War Museums. The outbreak of the First World War · Reputable sourceiwm.org.uk · The domain "iwm.org.uk" is on our Reputable source registry.
- National Army Museum. 1914: To Arms · Reputable sourcenam.ac.uk · The domain "nam.ac.uk" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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Related timelines
- World War I → · The full course of the war, from the trenches of the Western Front to the 1918 armistice, is covered on the World War I timeline.