The Adriatic Sea () is a body of water separating the Italian Peninsula from the Balkan Peninsula. The Adriatic is the northernmost arm of the Mediterranean Sea, extending from the Strait of Otranto (where it connects to the Ionian Sea) to the northwest and the Po Valley. The countries with coasts on the Adriatic are Albania, Bosnia and Herzegovina, Croatia, Italy, Montenegro, and Slovenia.

The Adriatic contains more than 1,300 islands, mostly located along the Croatian part of its eastern coast. It is divided into three basins, the northern being the shallowest and the southern being the deepest, with a maximum depth of 1,233 metres (4,045 ft). The Otranto Sill, an underwater ridge, is located at the border between the Adriatic and Ionian Seas. The prevailing currents flow counterclockwise from the Strait of Otranto, along the eastern coast and back to the strait along the western (Italian) coast. Tidal movements in the Adriatic are slight, although larger amplitudes are known to occur occasionally. The Adriatic's salinity is lower than the Mediterranean's because the Adriatic collects a third of the fresh water flowing into the Mediterranean, acting as a dilution basin. The surface water temperatures generally range from 30 C (86 F) in summer to 12 C (54 F) in winter, significantly moderating the Adriatic Basin's climate.

The Adriatic Sea sits on the Apulian or Adriatic Microplate, which separated from the African Plate in the Mesozoic era. The plate's movement contributed to the formation of the surrounding mountain chains and Apennine tectonic uplift after its collision with the Eurasian plate. In the Late Oligocene, the Italian Peninsula first formed, separating the Adriatic Basin from the rest of the Mediterranean. All types of sediment are found in the Adriatic, with the bulk of the material transported by the Po and other rivers on the western coast. The western coast is alluvial or terraced, while the eastern coast is highly indented with pronounced karstification. There are dozens of marine protected areas in the Adriatic, designed to protect the sea's karst habitats and biodiversity. The sea is abundant in flora and faunamore than 7,000 species are identified as native to the Adriatic, many of them endemic, rare and threatened ones.

The Adriatic's shores are populated by more than 3.5 million people; the largest cities are Bari, Venice, Trieste and Split. The earliest settlements on the Adriatic shores were Etruscan, Illyrian, and Greek. By the 2nd century BC, the shores were under Rome's control. In the Middle Ages, the Adriatic shores and the sea itself were controlled, to a varying extent, by a series of statesmost notably the Byzantine Empire, the Croatian Kingdom, the Republic of Venice, the Habsburg monarchy and the Ottoman Empire. The Napoleonic Wars resulted in the First French Empire gaining coastal control and the British effort to counter the French in the area, ultimately securing most of the eastern Adriatic shore and the Po Valley for Austria. Following Italian unification, the Kingdom of Italy started an eastward expansion that lasted until the 20th century. Following World War I and the collapse of Austria-Hungary and the Ottoman Empire, the entire eastern coast's control passed to Yugoslavia and Albania, except for Trieste and surrounding area which remained under Italian control. The former disintegrated during the 1990s, resulting in four new states on the Adriatic coast. Italy and Yugoslavia agreed on their maritime boundaries by 1975 and this boundary is recognised by Yugoslavia's successor states, but the maritime boundaries between Slovenia, Croatia, Bosnia and Herzegovina, and Montenegro are still disputed. Italy and Albania agreed on their maritime boundary in 1992.

Fisheries and tourism are significant sources of income all along the Adriatic coast. Adriatic Croatia's tourism industry has grown faster economically than the rest of the Adriatic Basin's. Maritime transport is also a significant branch of the area's economythere are 19 seaports in the Adriatic that each handle more than a million tonnes of cargo per year. The largest Adriatic seaport by annual cargo turnover is the Port of Trieste, while the Port of Split is the largest Adriatic seaport by passengers served per year.

Yugoslavia (; Serbo-Croatian: Jugoslavija / Југославија [juɡǒslaːʋija]; Slovene: Jugoslavija [juɡɔˈslàːʋija]; Macedonian: Југославија [juɡɔˈsɫavija]; lit. 'South Slavic Land') was a country in Southeast Europe and Central Europe for most of the 20th century. It came into existence after World War I in 1918 under the name of the Kingdom of Serbs, Croats and Slovenes by the merger of the provisional State of Slovenes, Croats and Serbs (which was formed from territories of the former Austro-Hungarian Empire) with the Kingdom of Serbia, and constituted the first union of the South Slavic people as a sovereign state, following centuries in which the region had been part of the Ottoman Empire and Austria-Hungary. Peter I of Serbia was its first sovereign. The kingdom gained international recognition on 13 July 1922 at the Conference of Ambassadors in Paris. The official name of the state was changed to Kingdom of Yugoslavia on 3 October 1929.

Yugoslavia was invaded by the Axis powers on 6 April 1941. In 1943, a Democratic Federal Yugoslavia was proclaimed by the Partisan resistance. In 1944 King Peter II, then living in exile, recognised it as the legitimate government. The monarchy was subsequently abolished in November 1945. Yugoslavia was renamed the Federal People's Republic of Yugoslavia in 1946, when a communist government was established. It acquired the territories of Istria, Rijeka, and Zadar from Italy. Partisan leader Josip Broz Tito ruled the country as president until his death in 1980. In 1963, the country was renamed again, as the Socialist Federal Republic of Yugoslavia (SFRY).

The six constituent republics that made up the SFRY were the SR Bosnia and Herzegovina, SR Croatia, SR Macedonia, SR Montenegro, SR Serbia, and SR Slovenia. Serbia contained two Socialist Autonomous Provinces, Vojvodina and Kosovo, which after 1974 were largely equal to the other members of the federation. After an economic and political crisis in the 1980s and the rise of nationalism, Yugoslavia broke up along its republics' borders, at first into five countries, leading to the Yugoslav Wars. From 1993 to 2017, the International Criminal Tribunal for the former Yugoslavia tried political and military leaders from the former Yugoslavia for war crimes, genocide, and other crimes committed during those wars.

After the breakup, the republics of Montenegro and Serbia formed a reduced federative state, the Federal Republic of Yugoslavia (FRY), known from 2003 to 2006 as Serbia and Montenegro. This state aspired to the status of sole legal successor to the SFRY, but those claims were opposed by the other former republics. Eventually, it accepted the opinion of the Badinter Arbitration Committee about shared succession and in 2003 its official name was changed to Serbia and Montenegro. This state dissolved when Montenegro and Serbia each became independent states in 2006, while Kosovo proclaimed its independence from Serbia in 2008.