when did britain declare war on germany

Britain has declared war on Germany twice in major 20th‑century conflicts: on 4 August 1914 at the start of World War I, and on 3 September 1939 at the start of World War II.

Quick answer

• In World War I, Britain declared war on Germany on 4 August 1914 , after Germany invaded neutral Belgium and ignored an ultimatum to withdraw.

• In World War II, Britain declared war on Germany on 3 September 1939 , after Germany invaded Poland and refused to halt its attack.

Which date people usually mean

Most casual queries about “when did Britain declare war on Germany” tend to refer to World War II , so the answer often given is 3 September 1939. However, historically there are two clear moments, so it is more precise to specify the war you are asking about.

Tiny timeline view

• 1 August 1914: Crisis in Europe escalates toward World War I.

• 4 August 1914: Britain declares war on Germany over the invasion of Belgium.

• 1 September 1939: Germany invades Poland.

• 3 September 1939: Britain (and France) declare war on Germany, marking the start of World War II in Europe.

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