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.