The First World War was truly ‘the Great War’. Its origins were complex. Its scale was vast. Its conduct was intense. Its impact on military operations was revolutionary. Its human and material costs were enormous. And its results wereprofound.The war was a global conflict. Thirty-two nations were eventually involved. Twenty-eight of these constituted the Allied and Associated Powers, whose principal belligerents were the British Empire, France, Italy, Russia, Serbia, and the United States of America. They were opposed by the Central Powers: Austria-Hungary, Bulgaria, Germany, and the Ottoman Empire.The war began in the Balkan cockpit of competing nationalisms and ancient ethnicrivalries. Hopes that it could be contained there proved vain. Expansion of the war was swift. Austria-Hungary declared war on Serbia on 28 July 1914; Germany declared war on Russia on 1 August. Germany declared war on France on 3 August and invaded Belgium. France was invaded on 4 August. German violation of Belgian neutrality provided the British with a convenient excuse to enter the war on the side of France and Russia the same evening. Austria-Hungary declared war on Russia on 6 August. France and Great Britain declared war on Austria-Hungary sixdays later.