1. Violation of Belgian Neutrality: The immediate trigger for Britain's entry into the war was Germany's invasion of Belgium on August 4, 1914. Britain had a long-standing commitment to protect Belgian neutrality, which had been guaranteed by the Treaty of London (1839). When Germany violated this treaty, Britain felt compelled to honor its commitment and protect Belgium.
2. Balance of Power: Britain's foreign policy was guided by the principle of maintaining a balance of power in Europe to ensure its own security. By 1914, Germany had emerged as a formidable military and economic power, challenging Britain's dominance. Britain feared that Germany's growing strength would upset the European balance of power, and an alliance with France and Russia was seen as a way to counter this threat.
3. Imperial Rivalry: Britain had a long-standing rivalry with Germany, particularly in terms of colonial expansion and economic competition. The two countries were vying for control of territories in Africa, the Middle East, and the Far East. By supporting France and Russia against Germany, Britain sought to weaken its rival and secure its own imperial interests.
4. French and Russian Alliance: Britain had established alliances with France and Russia prior to World War I. When Germany invaded Belgium, triggering France and Russia's mobilization, Britain felt obligated to uphold its treaty obligations and come to the aid of its allies.
5. Public Opinion: Public opinion in Britain was strongly in favor of supporting France and Belgium against Germany. The British public was outraged by Germany's invasion of Belgium and saw it as a clear violation of international law and morality. Anti-German sentiments were widespread, and the government felt compelled to respond to this public pressure.