The war drew in all the world's economic great powers, which were assembled in two opposing alliances: the Allies and the Central Powers of Germany and Austria-Hungary. Although Italy had also been a member of the Triple Alliance alongside Germany and Austria-Hungary, it did not join the Central Powers, as Austria-Hungary had taken the offensive against the terms of the alliance. These alliances were reorganised and expanded as more nations entered the war: Italy, Japan and the United States joined the Allies, and the Ottoman Empire and Bulgaria the Central Powers. Ultimately, more than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history.
The immediate trigger for war was the 28 June 1914 assassination of Archduke Franz Ferdinand of Austria, heir to the throne of Austria-Hungary, by Yugoslav nationalist Gavrilo Princip in Sarajevo. This set off a diplomatic crisis when Austria-Hungary delivered an ultimatum to the Kingdom of Serbia, and international alliances formed over the previous decades were invoked. Within weeks, the major powers were at war and the conflict soon spread around the world.
On 28 July, the Austro-Hungarians declared war on Serbia and subsequently invaded. As Russia mobilised in support of Serbia, Germany invaded neutral Belgium and Luxembourg before moving towards France, leading Britain to declare war on Germany. After the German march on Paris was halted, what became