On 4 August 1914, Britain declared war on Germany after a British ultimatum concerning Belgian neutrality expired without an acceptable reply.

German troops had crossed into Belgium that morning. Germany’s war plan required its armies to move through Belgian territory to attack France from the north. The Belgian government had rejected a German demand for unhindered passage and ordered its forces to resist.

Britain had spent the previous days deciding whether to enter the widening European war. Germany had declared war on Russia on 1 August and France two days later. France and Russia were allies, but Britain had no treaty that automatically required it to fight beside them.

The Liberal cabinet was divided. Some ministers believed Britain should remain outside a continental war unless British territory or direct interests were attacked. Others argued that a German defeat of France would threaten the balance of power and place the Channel coast under the control of a hostile state.

Britain had also given France assurances that the Royal Navy would protect its northern coast against a German attack. The fleet had been mobilised, but the government had not committed the army.

Belgium changed the political position. Britain, France, Prussia, Austria and Russia had signed the Treaty of London in 1839, which recognised Belgium as an independent and neutral state. The action required of each guarantor was open to argument, but the German invasion gave the government a clear public and parliamentary case for intervention.

On 3 August, Foreign Secretary Sir Edward Grey told the House of Commons that Britain could not ignore the consequences of France being defeated or Belgium being occupied. The cabinet authorised an ultimatum to Germany the following day.

Britain demanded an assurance that Germany would respect Belgian neutrality and withdraw its forces. The deadline was midnight in Berlin, which was 11 pm in London.

Germany did not provide an acceptable reply. When the deadline passed, Britain and Germany were at war.

The declaration applied across the British Empire. The self-governing Dominions decided how they would contribute, while India and the colonies came under the imperial state of war.

Britain began preparing the British Expeditionary Force for service in France. It was a small professional army compared with the mass conscript forces of the continental powers. Its first units crossed the Channel within days.