Belgium had been a part of the United Kingdom of the Netherlands since 1815 but by 1830, the Belgians broke away to form their own independent kingdom. After years of resistance from the Dutch and pushing by the Brits, they finally accepted Belgian independence by signing the Treaty of London in 1839. The British also insisted on Belgium’s neutrality, a fundamental part of the treaty especially in the lead up to World War I.
Germany demanded free passage through Belgian territory to attack France; this was denied by the Belgian government but ignored by the Germans when they invaded Belgium in August 1914. As a result, Britain declared war on Germany when Belgium's neutrality was violated. Henceforth, Britain and Belgium developed a brotherhood in arms which has stood the test of time.