mustered their strength and prepared to launch an attack to drive back the Turks from Thrace. In a crushing victory, the Ottomans annihilated the Serbian army at the Battle of Maritsa, and in its aftermath, many surviving lords submitted to the Ottoman Sultan Murad I. Byzantium was in no better position and after taking Serres from the defeated Serbs, John V swore allegiance as a vassal to Murad. Third Palaiologan Civil War, 1373–1379 John V's rule was an unhappy one, resulting in his vassalage to Murad I. However, it must have been all the more worse when his eldest son and heir to the throne Andronikos IV Palaiologos rebelled against his father in 1373. Curiously, this rebellion coincided with the rebellion of Murad I's son, Savci Celebi and the two worked towards fomenting revolution in their peoples. Consequently, both the Byzantine and Ottoman rulers were facing their sons and as a result, coordinated efforts were made to defeat both. John V had his eldest son, Andronikos IV, along with the latter's son, John VII, partially blinded, while Murad I defeated his son, Savci, and had him executed. Manuel, the second son of John V, was made co-emperor and heir to the throne. Unfortunately for John V, Andronikos IV and his son John VII escaped. With Genoan and Turkish aid, they returned to Constantinople and succeeded in overthrowing John V, imprisoning him and Manuel. In exchange for Ottoman aid, Andronikos IV handed the fortress of Gallipoli over to the Ottomans, thus rendering the only genuine European aid, provided by Amadeo of Savoy, useless. Again, prison break became the next event with John V and Manuel escaping Constantinople, offering the Ottoman Sultan higher tribute than normally paid, in return for help in taking over Constantinople. Andronikos IV, having been beaten yet again, evaded capture and slipped into the Genoese district of Galata with his family and hostages. John V, only interested in securing his throne and stability, came to conclude a pact with Andronikos IV in 1381, recognizing him as heir with John VII as heir apparent, thus removing Manuel from the line of succession. Naturally, Manuel felt betrayed by this move which demoted him from co-emperor. Returning to Thessalonica in 1382, he rebelled and established his rule over Thessaly and Epirus, thus "expanding" the Empire, at least nominally, and thereby catching the Ottoman Sultan's attention. Murad I besieged Thessalonica in 1383,