<P> The civil war did not end there; Matthew Cantacuzenus now obtained troops from Orhan and began a bid for taking Constantinople . His capture in 1356 ended his dreams of becoming Emperor and with it came an ephemeral defeat for the Ottomans who had favored the overthrow of John V . </P> <P> Following the end of the civil conflict came a small lull in fighting between the expanding Ottomans and Byzantines . In 1361 Didymoteichon fell to the Turks . Orhan's successor, Murad I was more concerned with his Anatolian positions . However, just like Alp Arslan of the Seljuk Turks, Murad I left the taking of Byzantine territory to his vassals with Philippopolis falling after major campaigning between 1363--4 and Adrianople succumbing to the Ottomans in 1369 . </P> <P> The Byzantine Empire was in no position to launch any decent counter-attack or defence of these lands; by now the Ottomans had become supremely powerful . Murad I crushed an army of Serbians on 26 September 1371 at the Battle of Maritsa leading to the end of Serbian power . The Ottomans were now poised to conquer Constantinople . In an attempt to stave off defeat, John V appealed to the Pope for support offering submission to Rome in return for military support . Despite publicly confessing the Roman Catholic Faith in St. Peter's Basilica, John V received no help . John V therefore was forced to turn to reason with his enemies, the Ottomans . Murad I and John V then came to an agreement whereby Byzantium would provide regular tribute in troops and money in exchange for security . </P> <P> By now the Ottomans had essentially won the war; Byzantium was reduced to a few settlements other than Constantinople and was forced to recognize its vassal status to the Ottoman Sultan . This vassalage continued until 1394 . However, whilst Constantinople had been neutralized, the surrounding Christian powers were still a threat to the Ottomans and Asia Minor was not under complete Ottoman control . The Ottomans continued their thrust into the Balkans, proving to be great conquerors in Europe as they were in Anatolia; in 1385 Sofia was captured from the Bulgarians and Niš was taken the following year . Other smaller states were subjugated as vassals, including the Serbs . Serbian resistance was crushed at the Battle of Kosovo in 1389, much of Bulgaria was taken in 1393 by Bayezid I (the Thunderbolt) and in 1396 the last bastion of Bulgarian independence was wiped out when Vdiin fell . </P>

In 1373 what was the byzantine emperor forced to do