World War I: Entente forces land in Thessaloniki, to open the Macedonian front against the Central Powers.

World War I or the First World War, also known as the Great War, was a global conflict between two coalitions: the Allies and the Central Powers. Main areas of conflict included Europe and the Middle East, as well as parts of Africa and the Asia-Pacific. There were important developments in weaponry including tanks, aircraft, artillery, machine guns, and chemical weapons. One of the deadliest conflicts in history, it resulted in an estimated 30 million military casualties, plus another 8 million civilian deaths from war-related causes and genocide. The movement of large numbers of people was a major factor in the deadly Spanish flu pandemic.

The Allies or the Entente was an international military coalition of countries led by the French Republic, the United Kingdom, the Russian Empire, the United States, the Kingdom of Italy, and the Empire of Japan against the Central Powers of the German Empire, Austria-Hungary, the Ottoman Empire, and the Kingdom of Bulgaria in World War I (1914–1918).

Thessaloniki is the second-largest city in Greece and the capital of the geographic region of Macedonia, the administrative region of Central Macedonia and the Decentralized Administration of Macedonia and Thrace. It is also known in Greek as i Symprotévousa, literally "the co-capital", a reference to its historical status as the "co-reigning" city of the Byzantine Empire alongside Constantinople.

The Macedonian front, also known as the Salonica front, was a military theatre of World War I formed as a result of an attempt by the Allied Powers to aid Serbia, in the autumn of 1915, against the combined attack of Germany, Austria-Hungary and Bulgaria. The expedition came too late and with insufficient force to prevent the fall of Serbia and was complicated by the internal political crisis in Greece. Eventually, a stable front was established, running from the Albanian Adriatic coast to the Struma River, pitting a multinational Allied force against the Bulgarian army, which was at various times bolstered with smaller units from the other Central Powers. The Macedonian front remained stable, despite local actions, until the Allied offensive in September 1918 resulted in Bulgaria capitulating and the liberation of Serbia.