Final answer:
The trigger of British involvement in World War I was Germany's violation of Belgium's neutrality, leading to Britain declaring war on Germany to uphold the Treaty of London and maintain European balance of power.
Step-by-step explanation:
The trigger of British involvement in World War I was the violation of Belgium's neutrality by Germany. On August 4, 1914, Britain declared war on Germany after the German Empire initiated an invasion of Belgium. This action contravened the Treaty of London (1839), which required Britain to defend Belgian neutrality in case of invasion. The Foreign Secretary, Edward Grey, had outlined the importance of reacting to Germany's aggression given Belgium's precarious position, making it clear that Britain could not ignore the breach of international law and the threat to European stability. As per the Schlieffen Plan, Germany intended to swiftly defeat France by moving through Belgium, drawing the British Empire into the conflict to safeguard Belgian sovereignty and maintain the balance of power in Europe. Initially, the British Expeditionary Force comprised nearly a quarter of a million soldiers to provide support to French forces resisting the German advance. The involvement of Great Britain in World War I was a matter of honoring treaties and responding to a perceived threat that the German aggression posed to European security and the interests of the British Empire.