British entry into World War I

The United Kingdom and the British Empire entered World War I on 4 August 1914, when King George V declared war after the expiry of an ultimatum to the German Empire. The official explanation focused on protecting Belgium as a neutral country; the main reason, however, was to prevent a French defeat that would have left Germany in control of Western Europe.

Source: Wikipedia — British entry into World War I (CC BY-SA 4.0)