The four main causes were the rise of nationalism, the building of armies and alliances, imperialism, and the assassination of Archduke Francis Ferdinand. Imperialism, as the conquest of nations, was aided by the establishment of a strong army and alliance and often led to the formation of nationalist groups in conquered colonies. One of these nationalist groups, the Black Hand, was responsible for the assassination of the Archduke, which would become the spark that would ignite the First World War. Shortly after the assassination, war was declared, and within a month Germany, Britain, France and Russia were all involved. By the end of the war, more than eight million people had died and World War I was known as one of the bloodiest wars in the world.