On 4 August 1914, 100 years ago today, Britain, following an ultimatum to Berlin, declared war on Germany. The July Crisis was over. The war became global.
Proclamation issued by the Foreign Office. London, 4 August 1914.
Owing to the summary rejection by the German Government of the request made by his Majesty’s Government for assurances that the neutrality of Belgium will be respected, his Majesty’s Ambassador to Berlin has received his passports, and his Majesty’s Government declared to the German Government that a state of war exists between Great Britain and Germany as from 11 p.m. on August 4, 1914.
Source: The Guardian. 5 August 1914.