Britain declares war on Germany

On August 4, 1914, Britain declared war on Germany effective 11 p.m. after Germany failed to respond to an ultimatum demanding respect for Belgian neutrality. Germany had invaded Luxembourg on August 2 and issued its own ultimatum to Belgium demanding military passage, which Belgium refused. Britain's obligation stemmed from the 1839 Treaty of London guaranteeing Belgian sovereignty. The declaration brought the British Empire, including its dominions and colonies, into the conflict.