1. In 1437, Demetrios Palaiologos accompanied his elder brother John VIII to the Council of Florence, the main objective of which was to unify the Roman Catholic and Eastern Orthodox churches so that Western Europe might be more inclined to lend military aid to the desperate Byzantine Empire.
Demetrios Palaiologos attempted to claim the throne twice, first attempting to take it with Ottoman support in 1442 and then by hoping to be proclaimed emperor after John VIII's death in 1448.
The constant disagreements between Thomas and Demetrios Palaiologos, who supported the Ottomans instead, eventually led Mehmed to invade and conquer the Morea in 1460.
Emperor Manuel II Demetrios Palaiologos had a total of six sons who survived infancy.
Demetrios Palaiologos was made the despot of the island of Lemnos in 1422 by Manuel; however, he was not satisfied and refused to live on the island.
In 1423, Demetrios Palaiologos fled to the court of King Sigismund of Hungary in 1423 under the pretext of needing protection against his brothers.
Demetrios Palaiologos was accompanied by two of his supporters, Hilario Doria and George Izaoul, and did not return to Constantinople until 1427.
Demetrios Palaiologos had just made a deal with Murad himself and had raised an army, portraying himself as the champion of the anti-unionist cause, which opposed the union of the Catholic and Eastern Orthodox Churches, a cause supported by the Turks, declaring war on his brother John VIII.
Demetrios Palaiologos was granted the despotate's nominal capital, Mystras, and primarily ruled the southern and eastern parts of the despotate, with Thomas ruling Corinthia and the north-west, variously using Patras or Leontari as his capital.
Theodora's brother Matthew Palaiologos Asen was appointed as governor of Corinth by Demetrios.
When emissaries of Thomas and Demetrios Palaiologos visited the Sultan at Adrianople some months after Constantinople's fall, the Sultan demanded no surrender of territory, only that the despots were to pay an annual tribute of 10,000 ducats.
However, Thomas and Demetrios Palaiologos were never able to cooperate and spent most of their resources fighting each other rather than preparing for a struggle against the Turks.
In return for the aid, Mehmed demanded a heavier tribute from Thomas and Demetrios Palaiologos, amounting to 12,000 ducats annually rather than the previous 10,000.
Demetrios Palaiologos raised the possibility of a marriage alliance with Alfonso of Aragon and Naples, suggesting that his daughter Helena marry Alfonso's grandson.
Thomas and Demetrios Palaiologos proved to be incapable of working together even with foreign diplomacy.
Demetrios Palaiologos had shifted to becoming even more pro-Ottoman after Mehmed had promised the despot that he would marry his daughter Helena, whereas Thomas increasingly hoped for western aid as the regions of the Morea annexed by Mehmed had been almost the entire area ruled by Thomas, including his capital of Patras.
Demetrios Palaiologos responded by seizing Leontari and called for aid from the Turkish governors in the northern Morea.
Lord Demetrios Palaiologos rested his hopes on the friendship and help of the sultan, and on his claim that his subjects and castles had been wronged, while Lord Thomas relied on the fact that his opponent had committed perjury and that he was waging war against the impious.
Demetrios Palaiologos surrendered to the Ottomans without a fight, fearing retribution and already having sent his family to safety in Monemvasia.
Demetrios Palaiologos was terrified of the sultan, but Mehmed received him kindly, offering him a chair and speaking gently to him.
Mehmed demanded that Demetrios Palaiologos recall his wife Theodora and his only child, Helena, from their refuge in Monemvasia and yield them to the sultan so that they could accompany him to Adrianople, but showered Demetrios Palaiologos with gifts and promised to grant him an appanage in Thrace as compensation for the loss of the Morea.
Theodora and Helena were put in the care of some of the eunuchs in Mehmed's entourage and Demetrios Palaiologos was obliged to accompany Mehmed as he went on to subdue the rest of the Morea after spending four days at Mystras.
Demetrios Palaiologos's appanage consisted of the islands Lemnos and Imbros, as well as parts of the islands Thassos and Samothrace as well as the town Enos in Thrace.
Demetrios Palaiologos lived at Enos for seven years, together with his wife Theodora and her brother, Matthew Palaiologos Asen.
In 1467, George Sphrantzes, former companion to Constantine XI, who had little regard for either Matthew or Demetrios Palaiologos, wrote that Matthew, who controlled a local salt monopoly, had allowed his underlings to cheat the government on some revenue.