August 4, 1914 – The First World War, or the Great War, begins when Germany invades neutral Belgium, prompting a declaration of war from allies Britain, France and Russia. Australia and New Zealand also pledge their support and begin calling for volunteers. The Australian Imperial Force (AIF) is born.
This global war is centered in Europe where more than 70 million military personnel are mobilised. Over 9 million combatants and 7 million civilians die as a result of the war.
The trigger for the war is the assassination of Archduke Franz Ferdinand of Austria, heir to the throne of Austria-Hungary, by Yugoslav nationalist Gavrilo Princip in Sarajevo on 28 June 1914. This set off a diplomatic crisis when Austria-Hungary delivers an ultimatum to the Kingdom of Serbia, and entangled international alliances formed over the previous decades were invoked. Within weeks, the major powers are at war and the conflict soon spreads around the world.
On 28 July, the Austro-Hungarians declare war on Serbia and subsequently invaded. As Russia mobilises in support of Serbia, Germany invades neutral Belgium and Luxembourg before moving towards France, leading the United Kingdom to declare war on Germany. After the German march on Paris is halted, what became known as the Western Front settles into a battle of attrition, with a trench line that changes little until 1917. Meanwhile, on the Eastern Front, the Russian army is successful against the Austro-Hungarians, but is stopped in its invasion of East Prussia by the Germans. In November 1914, the Ottoman Empire joins the Central Powers, opening fronts in the Caucasus, Mesopotamia and the Sinai. Italy joins the Allies in 1915 and Bulgaria joins the Central Powers in the same year, while Romania joins the Allies in 1916, followed by the United States in 1917.
By the end of the war, the German Empire, Russian Empire, Austro-Hungarian Empire and the Ottoman Empire cease to exist. National borders are redrawn, with several independent nations restored or created, and Germany’s colonies parcelled out among the winners.
During the Paris Peace Conference of 1919, the Big Four (Britain, France, the United States and Italy) impose their terms in a series of treaties. The League of Nations forms with the aim of preventing any repetition of such a conflict. This effort failed, and economic depression, renewed European nationalism, weakened member states, and the German feeling of humiliation contributed to the rise of Nazism. These conditions eventually contribute to World War II.