The Balkan Wars were two conflicts that took place in the Balkan Peninsula in south-eastern Europe in 1912 and 1913.

By the early 20th century, Montenegro, Bulgaria, Greece and Serbia had achieved independence from the Ottoman Empire of Turkey. But large parts of their ethnic populations remained under Ottoman rule. In 1912, these countries formed the Balkan League. The First Balkan War broke out when the League attacked Turkey on 8 October 1912 and was ended seven months later by the Treaty of London. After five centuries, Turkey lost virtually all of its possessions in the Balkans.

The Second Balkan War broke out on 16 June 1913. Bulgaria was dissatisfied over the division of the spoils in Macedonia, made in secret by its former allies, Serbia and Greece, and attacked them. The Serbian and Greek armies repulsed the Bulgarian offensive and counter-attacked into Bulgaria, while Romania and Turkey also attacked Bulgaria and gained (or regained) territory. In the resulting Treaty of Bucharest, Bulgaria lost most of the territories it had gained in the First Balkan War.

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.