The United Kingdom 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. The Liberal Party was in power with prime minister H. H. Asquith and foreign minister Edward Grey leading the way. The Liberal cabinet made the decision, although the party had been strongly anti-war until the last minute. The Conservative Party was pro-war. The Liberals knew that if they split on the war issue, they would lose control of the government to the Conservatives.
Background
For much of the 19th century, Britain pursued a foreign policy later known as splendid isolation, which sought to maintain the balance of power in Europe without formal alliances. As Europe divided into two power blocs during the 1890s, the 1895-1905 Conservative government realised this left Britain dangerously exposed. This resulted in the 1902 Anglo-Japanese Alliance, followed by King Edward VII's 1903 visit to Paris. By reducing anti-British feeling in France, it led to the 1904 Entente Cordiale, the first tangible impact of which was British support for France against Germany in the 1905 Moroccan Crisis.
In 1907, the new Liberal government agreed to the Anglo-Russian Convention. Like the Entente, the Convention focused on resolving colonial disputes; but by doing so, it paved the way for wider co-operation and allowed Britain to refocus its naval resources in response to German naval expansion.
The 1911 Agadir Crisis encouraged secret military negotiations between France and Britain in the case of war with the German Empire. A British Expeditionary Force of 100,000 men would be landed in France within two weeks of war, while naval arrangements allocated responsibility for the Mediterranean Sea to the French Navy, with the Royal Navy looking after the North Sea and the English Channel, including Northern France. Britain was effectively bound to support France in a war against Germany regardless, but this was not widely understood outside government and the military.
Antagonism with Germany
In explaining why Britain went to war with Germany, British historian Paul Kennedy (1980) argued that a critical factor was the British realisation that Germany was rapidly becoming economically more powerful than Britain. It was in essence not a consequence of the disputes over economic trade imperialism, the Baghdad Railway, the confrontations in Eastern Europe, highly-charged political rhetoric, or domestic pressure groups. Germany's reliance time and again on military aggression, while Britain increasingly appealed to moral sensibilities, also played a role, especially in portraying the invasion of neutral Belgium as (in the German view) a necessary military tactic or (in the British view) a profound moral crime. The German invasion of Belgium was not the real cause of war with Britain, because the British decision had already been made as the British were more concerned with the fate of France (pp. 457–62). Kennedy argues that by far the main reason was London's fear that a repeat of the war of 1870, when Prussia and the German states smashed France, would mean that a rapidly industrialising Germany, with a powerful army and navy, would control the English Channel and northwest France. British policy-makers insisted that that would be a catastrophe for British security.
Christopher Clark points out that the British cabinet decided on July 29, 1914, that being a signatory to the 1839 treaty guaranteeing Belgium's frontiers did not oblige it to oppose a German invasion of Belgium with military force.
Decision for war
In the immediate aftermath of the assassination on June 28 of Austrian Archduke Franz Ferdinand (the heir to the Habsburg throne) in the Bosnian capital, Sarajevo, the British newspapers denounced the Serbian nationalist assassin, Gavrilo Princip, and were generally sympathetic to the Austro-Hungarian monarchy. The newspapers blamed the Kingdom of Serbia for the crime, with rhetoric against "fanatics", "dangerous forces" and "reckless agitators". These responses were broadly shared across the political spectrum, with Liberal and Conservative papers expressing their shock and dismay. But by July 27, press opinion had turned against Austria-Hungary. The national press divided along party lines, with Conservative papers stressing the obligation to support France, while Liberal papers insisted Britain had no such commitment and should remain neutral.