The United Kingdom and the British Empire declared war on Austria-Hungary in response to the ongoing tensions sparked by the assassination of Archduke Franz Ferdinand of Austria in June 1914. The archduke’s death triggered a series of diplomatic crises and treaties that ultimately led to a widespread conflict. Following a request for intervention from Serbia, which was facing an ultimatum from Austria-Hungary, the UK intervened to protect its interests and maintain the balance of power in Europe. The British government’s decision was influenced by their obligations to support France and Russia, both allies of Serbia, in this fragile geopolitical landscape.

On 12 August 1914, the British Prime Minister, Herbert Henry Asquith, announced the declaration of war. This marked the beginning of the UK's full-scale military involvement in World War I, representing a significant escalation in the conflict. The United Kingdom’s entry into the war had regained not just military support for Serbia, but a promise to maintain the stability of Europe against aggressive movements from Austria-Hungary and its ally, Germany. The UK's declaration of war rallied the support of its colonies and dominions around the globe, exemplifying the vast reach of the British Empire at the time.

As the war unfolded, it became clear that alliances formed prior to the war played a pivotal role in the conflict's progression. The declaration set the stage for a multi-nation confrontation, where millions would be involved in one of the largest and deadliest wars in history. The UK's military efforts were essentially coordinated with other allies and would dictate several strategic decisions throughout the course of the war.