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Early reign

Serbia made some raids into the Macedonia region in late 1331, but a planned major attack on Byzantium was delayed as Dušan had to suppress revolts in Zeta in 1332. Dušan's ingratitude toward those who had aided his rise – the Zetan nobility may have been neglected their promised reward and greater influence – may have been the cause of the rebellion, which was suppressed in the course of the same year.

The struggle between Dušan and Stefan Dečanski prevented Serbian intervention in Bulgaria on behalf of Anna and Ivan Stephen and they were overthrown by Ivan Alexander in March 1331, however Ivan Alexander of Bulgaria was aware of future danger from Serbia and immediately sought peace with Dušan. Two rulers concluded peace and formed an alliance, sealed with Dušan's marriage to Ivan Alexander's sister Helen of Bulgaria. Good relations with Bulgaria continued during Dušan's reign and though Bulgaria was weaker then Serbia, it did not suffer any legal dependence.

In 1333 after negotiations with Ragusa, Dušan sold Ston and its envisions, including the Pelješac peninsula and the coastland between Ston and Dubrovnik for eight thousand perpers and an annual tribute of five hundred perpers. Ragusa also had to guarantee freedom of worship for Orthodox believers in this territory.

Dušan began campaigning against the Byzantine Empire in 1333. In late 1333, a leading Byzantine general Syrgiannes revolted against Andronikos III and sought Dušan's help. In spring of 1334 Serbs launched an attack on Byzantine Macedonia, benefiting greatly due to Syrgiannes' strategic abilities, knowledge of Byzantine position and his allies that surrendered fortresses to Serbs. Peace with Byzantines was concluded on 26. August 1334, with Byzantines recognizing Serbian gains in Ohrid, Prilep, Strumitsa, Siderokastron, Chermen and Prosek. Hungarians, knowing of Dušan's involvement in the south, were mobilizing to attack Serbia from the north. Hungarians, not expecting any serious Serbian resistance penetrated deep into Serbia, reaching neighborhood of Žiča monastery. Hungarians have received military support from Stephen II of Bosnia. Dušan marched north to face Hungarians. Hungarians quickly withdrew their armies to avoid Dušan's army. Charles I was wounded by an arrow but survived. As a result, the Hungarians lost Mačva and Belgrade in 1335. Dušan then focused his attention on the internal affairs of his country, writing, in 1349, the first statute book of the Serbs.

Dušan exploited the civil war in the Byzantine Empire between the regent of the minor Emperor John V Palaiologos, Anna of Savoy, and his father's general John Kantakouzenos. Dušan and Ivan Alexander picked opposite sides in the conflict but remained at peace with each other, taking advantage of the Byzantine civil war to secure gains for themselves.

Dušan's systematic offensive began in 1342, and in the end he conquered all Byzantine territories in the western Balkans as far as Kavala, except for the Peloponnesus and Thessaloniki, which he could not besiege due to his small fleet. There has been speculation that Dušan's ultimate goal was no less than to conquer Constantinople and replace the declining Byzantine Empire with a united Orthodox Greco-Serbian Empire under his control. In May 1344, his commander Preljub was stopped at Stephaniana by a Turkic force of 3,100. The Turks won the battle, but the victory was not enough to thwart the Serbian conquest of Macedonia. Faced with Dušan's aggression, the Byzantines sought allies in the Ottoman Turks, whom they brought into Europe for the first time.

In 1343, Dušan added "of Romans (Greeks)" to his self-styled title "King of Serbia, Albania and the coast". In another instance, in a charter issued to the fortified city of Krujë, Dušan referred to himself as "King of the Bulgarians". In 1345 he began calling himself tsar, equivalent of Emperor, as attested in charters to two athonite monasteries, one from November 1345 and the other from January 1346, and around Christmas 1345 at a council meeting in Serres, which was conquered on 25 September 1345, he proclaimed himself "Tsar of the Serbs and Romans" (Romans is equivalent to Greeks in Serbian documents).