The Treaty of Versailles (French: le Traité de Versailles) was one of the peace treaties at the end of World War I. It ended the state of war between Germany and the Allied Powers. It was signed on 28 June 1919, exactly five years after the assassination of Archduke Franz Ferdinand. The other Central Powers on the German side of World War I were dealt with in separate treaties. Although the armistice signed on 11 November 1918, ended the actual fighting, it took six months of negotiations at the Paris Peace Conference to conclude the peace treaty. The treaty was registered by the Secretariat of the League of Nations on 21 October 1919, and was printed in The League of Nations Treaty Series.

Of the many provisions in the treaty, one of the most important and controversial required Germany to accept responsibility for causing the war (along with Austria and Hungary, according to the Treaty of Saint-Germain-en-Laye and the Treaty of Trianon, respectively) and, under the terms of articles 231–248 (later known as the War Guilt clauses), to disarm, make substantial territorial concessions and pay heavy reparations to certain countries that had formed the Entente powers. In 1921 the total cost of these reparations was assessed at 132 billion Marks (then $31.4 billion or £6.6 billion, roughly equivalent to US $442 billion or UK £284 billion in 2012), a sum that many economists at the time, notably John Maynard Keynes, deemed to be excessive and counterproductive and would have taken Germany until 1988 to pay. The final payments ended up being made on 4 October 2010, the 20th anniversary of German reunification, and some 92 years after the end of the war for which they were exacted. The Treaty was undermined by subsequent events starting as early as 1932 and was widely flouted by the mid-1930s.

World War I (WWI), which was predominantly called the World War or the Great War from its occurrence until 1939 (World War II), and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. It involved all the world's great powers, which were assembled in two opposing alliances: the Allies (based on the Triple Entente of the United Kingdom, France and Russia) and the Central Powers (originally centred around the Triple Alliance of Germany, Austria-Hungary and Italy; but, as Austria–Hungary had taken the offensive against the agreement, Italy did not enter into the war). These alliances both reorganised (Italy fought for the Allies), and expanded as more nations entered the war. Ultimately more than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history. More than 9 million combatants were killed, largely because of enormous increases in lethality of weapons, thanks to new technology, without corresponding improvements in protection or mobility. It was the sixth-deadliest conflict in world history, subsequently paving the way for various political changes such as revolutions in the nations involved.