Why did Britain declare war against the central powers at the start of world war 1?

Britain declared war against the Central Powers on August 4, 1914, primarily due to Germany's violation of Belgium's neutrality. The Treaty of London (1839) had committed Britain to protect Belgium, and when Germany invaded Belgium as part of its military strategy, Britain saw this as a direct threat to its national security and international obligations. Additionally, concerns over maintaining the balance of power in Europe and protecting its allies, particularly France, further compelled Britain to enter the war.