The First World War, or World War I, as it was also known as, was a global war centred in Europe that was declared on July 28 1914, and lasted right through until 11 November 1918.

The trigger for the war had been pulled just prior to the declaration in July of 1914. On June 28 of that same year, there was an assassination of Archduke Franz Ferdinand of Austria, heir to the throne of Austria-Hungary. It is recorded that this was carried out by Yugoslav nationalist Gavrilo Princip in Sarajevo. This set off a diplomatic crisis when Austria-Hungary delivered the 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.

That next month, just as war was declared on July 28, the Austro-Hungarians declared war on Serbia and subsequently invaded. As Russia mobilised in support of Serbia, Germany invaded neutral Belgium and the little country of Luxembourg before moving towards France, leading Britain to declare war on Germany. After the German march on Paris was halted, what became known as the Western Front settled into a battle of attrition, with a trench line that would change little until 1917. Meanwhile, on the Eastern Front, the Russian army was successful against the Austro-Hungarians, but was stopped in its invasion of East Prussia by the Germans. In November 1914, the Ottoman Empire joined the Central Powers, opening fronts in the Caucasus, Mesopotamia and the Sinai. Italy joined the Allies in 1915 and Bulgaria joined the Central Powers in the same year, while Romania joined the Allies in 1916, and the United States joined the Allies in 1917.

The war approached a resolution after the Russian government collapsed in March 1917, and brought the Russians to terms with the Central Powers. On a subsequent revolution in November, the Austro-Hungarian empire agreed to an armistice. After a 1918 German offensive along the western front, the Allies drove back the Germans in a series of successful offensives and began entering the trenches. Germany, which had its own trouble with revolutionaries, agreed to an armistice on 11 November 1918, ending the war in victory for the Allies.

More than 9 million combatants and 7 million civilians died as a result of the war. It was one of the deadliest conflicts in history, paving the way for major political changes, including revolutions in many of the nations involved.

By the end of the war, four major imperial powers—the German, Russian, Austro-Hungarian and Ottoman empires—ceased to exist. The maps of Europe and Southwest Asia were redrawn, with several independent nations restored or created. The Big Four imposed their terms in a series of treaties. The League of Nations was formed with the aim of preventing any repetition of such an appalling conflict. This aim, however, failed with weakened states, renewed European nationalism and the German feeling of humiliation contributing to the rise of fascism. All of these conditions eventually led to World War II.