Thessaloniki (; Greek: , [esalonici] (listen)), also known as Thessalonica (English: ), Saloniki or Salonica () is the second-largest city in Greece, with over one million inhabitants in its metropolitan area, 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 Symprotvousa), literally "the co-capital", a reference to its historical status as the (Symvasilvousa) or "co-reigning" city of the Byzantine Empire alongside Constantinople.Thessaloniki is located on the Thermaic Gulf, at the northwest corner of the Aegean Sea. It is bounded on the west by the delta of the Axios. The municipality of Thessaloniki, the historical center, had a population of 317,778 in 2021, while the Thessaloniki metropolitan area had 1,030,338 inhabitants in 2011. It is Greece's second major economic, industrial, commercial and political centre, and a major transportation hub for Greece and southeastern Europe, notably through the Port of Thessaloniki. The city is renowned for its festivals, events and vibrant cultural life in general, and is considered to be Greece's cultural capital. Events such as the Thessaloniki International Fair and the Thessaloniki International Film Festival are held annually, while the city also hosts the largest bi-annual meeting of the Greek diaspora. Thessaloniki was the 2014 European Youth Capital.The city was founded in 315 BC by Cassander of Macedon, who named it after his wife Thessalonike, daughter of Philip II of Macedon and sister of Alexander the Great. An important metropolis by the Roman period, Thessaloniki was the second largest and wealthiest city of the Byzantine Empire. It was conquered by the Ottomans in 1430 and remained an important seaport and multi-ethnic metropolis during the nearly five centuries of Turkish rule. It passed from the Ottoman Empire to the Kingdom of Greece on 8 November 1912. Thessaloniki exhibits Byzantine architecture, including numerous Paleochristian and Byzantine monuments, a World Heritage Site, as well as several Roman, Ottoman and Sephardic Jewish structures. The city's main university, Aristotle University, is the largest in Greece and the Balkans.Thessaloniki is a popular tourist destination in Greece. In 2013, National Geographic Magazine included Thessaloniki in its top tourist destinations worldwide, while in 2014 Financial Times FDI magazine (Foreign Direct Investments) declared Thessaloniki as the best mid-sized European city of the future for human capital and lifestyle.

The First Balkan War (Serbian: Први балкански рат, Prvi Balkanski rat; Bulgarian: Балканска война; Greek: Αʹ Βαλκανικός πόλεμος; Turkish: Birinci Balkan Savaşı) lasted from October 1912 to May 1913 and involved actions of the Balkan League (the Kingdoms of Bulgaria, Serbia, Greece and Montenegro) against the Ottoman Empire. The Balkan states' combined armies overcame the initially numerically inferior (significantly superior by the end of the conflict) and strategically disadvantaged Ottoman armies and achieved rapid success.

The war was a comprehensive and unmitigated disaster for the Ottomans, who lost 83% of their European territories and 69% of their European population. As a result of the war, the League captured and partitioned almost all of the Ottoman Empire's remaining territories in Europe. Ensuing events also led to the creation of an independent Albania, which angered the Serbs. Bulgaria, meanwhile, was dissatisfied over the division of the spoils in Macedonia, and attacked its former allies, Serbia and Greece, on 16 June 1913 which provoked the start of the Second Balkan War.