While the Ottomans had been expanding out of Anatolia a different force had been growing in the north to challenge the Byzantine Empire. In 1331 Stephen Dushan ascended the throne of Serbia and spent the next 20 years building up a Serbian Empire. Just like the Ottomans Dushan had taken full advantage of the Byzantine succession dispute to conquer much of Albania as well as parts of Thrace and Macedonia. In 1346 he had himself crowned 'Emperor of the Serbs and Greeks', and invited the Venetians to join him in the conquest of Constantinople. Rumours that he was interested in reuniting the Catholic and Orthodox confessions ensured Papal support for the scheme, but in 1355, just as he was about to set off on a grand crusade, Stephen Dushan died.
Dushan's Serbian Empire rapidly started to crumble after his death, but when the Ottomans occupied Philippopolis in 1363 there was sufficient glory left in the Serbian name to persuade its defeated commander to seek help from that direction. A united force of Serbs, Bosnians and Wallachians joined a Hungarian army under the Hungarian king, Louis the Great, and marched against the Turks at Edirne. But their rapid advance made the crusaders lazy. Less than two days from Edirne they made camp on the banks of the river Maritza and celebrated their progress with feasting. The local Ottoman commander led his predominately light cavalry arm in an ambush by night. The Christians fled across the Maritza, which was in spate, and thousands drowned.
In 1365 the Sultan transferred his capital from Bursa to Edirne, a move of tremendous significance. To locate one's capital on the edge of one's territory next to hostile neighbours was an act of enormous self-confidence, and it proclaimed the sultan's future intentions with profound clarity, Edirne was also a natural centre of horse-breeding and soon became the seat of the imperial stables and stud-farms for the cavalry. Long after the capture of Constantinople it remained a favourite imperial residence.
From Edirne Murad I could look out over his territory as far as the coast of the Black Sea, a stretch of land that encircled the rapidly decreasing area dependent upon Constantinople. The toehold in Europe established at Gelibolu had now been replaced by a mighty presence and a dramatic statement of intent. The Ottoman advance could now continue from a firm base. The greatest phase of the conquests was about to start.