Serbia in the Middle Ages had an army that was well known for its strength and was among the strongest in the Balkans prior to the Ottoman conquest of Europe. Prior to the 14th century, the army consisted of European-style noble cavalry armed with bows and lances (replaced with crossbows in the 14th century) and infantry armed with spears, javelins and bows. With the economic growth from mining, mercenary knights from Western Europe were recruited to finalize and increase the effectiveness of the army, especially throughout 14th century.

Between 839 and 842 the Bulgar Khan Presian invaded inland parts of the Serbian territory, the two having lived peacefully sharing a common frontier up until that point. The cause for the invasion is unclear. It led to a war that lasted three years, in which the Bulgars were decisively defeated. In the mid 850s the Bulgarians under Boris I made another unsuccessful attempt to subdue the Serbs. In the beginning of the 10th century Simeon I launched several campaigns against the Serbs who were acting as Byzantine allies and by 925 he managed to conquer Serbia completely but the Bulgarian rule was short-lived. Samuel of Bulgaria subjected the Serbs for a second time in 1009 or 1010 after he defeated their ruler Jovan Vladimir.

10th-century Byzantine military manuals mention chonsarioi, light cavalry formations recruited in the Balkans, especially Serbs, "ideal for scouting and raiding". These units are considered a precursor of Hussar cavalry formations later found in Hungarian and Polish armies.

The writings of John Kinnamos and other contemporary Byzantine sources (Anna Komnene, Niketas Choniates, Eustathios of Thessalonica, Michael of Thessalonica) of the twelfth century, gives substantial data concerning Serbian armament and tactics.

Serbia defeated the Bulgarian Empire in the Battle of Velbazhd in 1330, becoming a military superpower during the middle part of the 14th century under the dynamic Stephen Uroš IV Dušan (1331–55), who created the Serbian Empire. It included Macedonia, Albania, Epirus and Thessaly, reaching from the Drina and Danube rivers as far south and east as the Gulf of Patras and the Rhodope Mountains by 1350. A large part of this expansion was at the expense of the Byzantine Empire. Dušan set his sights on the Byzantine capital of Constantinople itself, dividing his lands into 'Serbia' and 'Romania'. Dušan was crowned Emperor (Tsar) of Serbs and Romans in 1346. He modeled his court on that of Constantinople, calling his officials by Byzantine titles such as caesar, despot, sebastokrator and logotet (logothete). However, following his death, the Serbian Empire gradually disintegrated under his successor Stephen Uroš V (1355–71). One Byzantine chronicler noted with evident satisfaction that the Serbian nobility were soon divided into '10,000 factions', while John VI Kantakouzenos wrote that Dušan's empire fell 'into a thousand pieces'.

After 1363, Vukašin Mrnjavčević became the most powerful noble in the Serbian Empire; he controlled lands in the south of the empire, primarily in Macedonia. He was defeated and killed by the Ottoman Turks in 1371 in the Battle of Marica. In 1371, Prince Lazar Hrebeljanović emerged as the most powerful Serbian lord. He created the largest state on the territory of the disintegrated Serbian Empire; his state is known in historiography as Moravian Serbia. Its government and army were better organized than those of the domains of the other Serbian lords. In 1386, Prince Lazar rebuffed the Ottoman Sultan Murad I at Pločnik, a site southwest of the city of Niš.

An Ottoman army led by Sultan Murad, estimated at between 27,000 and 30,000 men, arrived in June 1389 on the Kosovo Field near Priština. The Ottomans were met by the forces commanded by Prince Lazar, estimated at between 15,000 and 20,000 men, with a higher estimate up to 25,000, A higher estimate places the size of Murad's army up to 40,000 and Lazar's up to 25,000 troops. which consisted of the prince's own troops, Vuk Branković's troops, and a contingent sent by the King Tvrtko I of Bosnia. In the Battle of Kosovo, both Prince Lazar and Sultan Murad lost their lives. The battle was tactically inconclusive, but the mutual heavy losses were devastating only for the Serbs, who had brought to Kosovo almost all of their fighting strength. Lazar was succeeded by his eldest son Stefan Lazarević, who became an Ottoman vassal in the summer of 1390. Vuk Branković accepted Ottoman suzerainty in 1392. The battle of Kosovo was one of the large battles of late medieval times. In comparison, in the battle of Agincourt (1415) even by assuming the higher estimate of army size as correct, around 10,000 fewer soldiers were engaged. Later, Stefan Lazarević participated on the Ottoman side in the Battle of Rovine in 1395, the Battle of Nicopolis in 1396, and the Battle of Ankara in 1402.

Stefan Lazarević was granted the title of Despot by the Byzantine Emperor, and he ceased to be an Ottoman vassal in 1402. He introduced modern western style knight warfare and knight tournaments. And there were firearms already in use, especially in armored cavalry. He ruled Serbia as Despot from 1402 to 1427. He was fighting against the Ottomans, and later he supported Mehmet I rise to power in the Battle of Çamurlu.