The causes of Britain's entry into the First World War combine long-term structural factors with the specific trigger of the July Crisis of 1914. Examiners expect you to distinguish between the underlying causes and the immediate trigger.
Long-term: The alliance system created a tripwire — By 1914, Europe was divided into two armed blocs. Any conflict between a major power and a smaller state could drag in all the great powers through their alliances. The alliances had been designed to deter war by making it too costly — instead they guaranteed that a local conflict (Austria vs Serbia) would become a continental war. Britain's entente with France and Russia did not formally commit it to war, but it made neutrality politically difficult once France was under attack.
Long-term: The arms race created fear and suspicion — Britain and Germany had been locked in a naval arms race since the late 1890s. Germany's rapid naval expansion challenged British supremacy at sea — a direct threat to an island nation that depended on sea trade for food and raw materials. By 1914, Britain had built the Dreadnought programme to maintain its lead, but mutual fear and suspicion made compromise impossible. This made war feel increasingly inevitable to British strategists even before the July Crisis.
Immediate trigger: The assassination of Franz Ferdinand — The murder of Archduke Franz Ferdinand in Sarajevo on 28 June 1914 set off the alliance tripwire. Austria's ultimatum to Serbia, Serbia's partial acceptance, Austria's declaration of war, Russia's mobilisation, and Germany's declarations of war on Russia and France all occurred within five weeks. Britain did not have to enter — but the logic of the alliances and the threat to France made neutrality increasingly costly.
The Belgian guarantee was the decisive factor for Britain — Germany's invasion of neutral Belgium on 4 August 1914 transformed the British debate. Before Belgium, neutrality had significant support in the Cabinet and the Liberal Party. The violation of the 1839 Treaty of London — which Britain had signed guaranteeing Belgian neutrality — gave Britain both a legal obligation and a clear moral argument. It also meant that entering the war was far easier to justify to the British public. Several Cabinet ministers who had threatened to resign if Britain entered the war stayed when Belgium changed the moral picture.
= Britain's decision was not inevitable — it was the product of specific choices — Britain could have remained neutral in 1914. The alliance with France and Russia was an entente, not a formal military commitment. The decision to go to war was made by the Cabinet, led by Foreign Secretary Sir Edward Grey. Without Belgium, it might not have happened. But strategic interest (not wanting German domination of Europe), alliance loyalty, and the moral shock of Belgium made war the chosen path.