In the hopes of raising support for a crusade to restore his lands in the Morea, and possibly the Byzantine Empire itself, Thomas travelled to Rome, where he was received and provided for by Pope Pius II.

After his death, his claims were inherited by his oldest son Andreas, who also attempted to rally support for a campaign to restore the fallen despotate and the Byzantine Empire.

As the Byzantine Empire fell apart and fragmented over the course of the 14th century, the emperors of the Palaiologan dynasty came to feel that the only sure way to keep their remaining holdings intact was to grant them to their sons, receiving the title of despot, as appanages to defend and govern.

[3] Shortly after being appointed as despots, Constantine and Thomas, together with Theodore, decided to join forces in an attempt to seize the flourishing and strategic port of Patras in the north-west of the Morea, then under the rule of its Catholic Archbishop.

By the 1430s, Thomas and Constantine had ensured that nearly the entire Peloponnese was once more in Byzantine hands for the first time since 1204, the only exception being the few port towns and cities held by the Republic of Venice.

In 1431, Turahan Bey, a Turkish general who governed Thessaly, sent his troops south to demolish the Morea's primary defensive fortifications, the Hexamilion wall, in an effort to remind the despots that they were the Sultan's vassals.

[8] With Theodore and Demetrios out of their way, Constantine and Thomas hoped to strengthen the Morea, by now the cultural center of the Byzantine world, and make it a safe and nearly self-sufficient principality.

[12] Although Turahan failed to take Mystras, this was of little consequence as Murad did not wish to conquer the Morea at the time, merely to instill terror, and the Turks soon left the peninsula, devastated and depopulated.

[20] In 1452, during the preparation stages of the Ottoman siege of Constantinople, Constantine XI sent an urgent message to the Morea, requesting that one of his brothers bring their forces to help him defend the city.

[24] Some of these influential refugees and courtiers even raised the idea of proclaiming Demetrios, the elder brother, as the Emperor of the Romans and the legitimate successor of Constantine XI.

[20] Shortly after Constantinople fell, a revolt broke out against the despots in the Morea, prompted by the many Albanian immigrants to the region being unhappy with the actions of the local Greek landowners.

The response was enthusiastic; some of Europe's most powerful and influential rulers came forward to take the cross, including Philip the Good of Burgundy in February 1454 and Alfonso the Magnanimous of Aragon and Naples in November 1455.

At Frankfurt, Holy Roman Emperor Frederick III assembled a council of German princes and proposed that 40,000 men be sent to Hungary, where the Ottomans had suffered a crushing defeat at Belgrade in 1456.

If the combined forces of Hungary, Aragon, Burgundy and the Holy Roman Empire had been unleashed to exploit the victory at Belgrade, Ottoman control of the Balkans would have been seriously threatened.

[26] Despite the Ottomans having secured the position of the two despots in the recent Albanian uprising, the possibility of Western aid to restore Byzantine territory proved too enticing to resist.

From Rome, Argyropoulos also moved on to Milan, England and France and further envoys were sent to Aragon (because of Alfonso's involvement in the crusading plans) and Venice (since Thomas were hoping that he could secure refuge in Venetian territory in the event of an Ottoman attack on the Morea).

[21] Leaving his artillery to bombard and besiege that city, Mehmed left with most of his army to devastate and conquer the northern parts of the despotate, under Thomas's jurisdiction.

The territory under the two brothers was drastically reduced, Corinth, Patras and much of the north-west of the peninsula were annexed into the Ottoman Empire and provided with Turkish governors, with the Palaiologoi only being allowed to keep the south, including the despotate's nominal capital, Mystras, on the condition that they paid their annual tribute to the sultan.

After a successful skirmish against the Ottomans, Thomas sent 16 captured Turkish soldiers, alongside some of his armed guards, to Rome to convince the Pope that he was engaging in a holy war against the Muslims.

With these reinforcements, Thomas gained the upper hand and it looked as if Demetrios was about to be defeated, having retreated to the town of Monemvasia and having sent Matthaios Asan to Adrianople to beg Mehmed for aid.

As large numbers of Greek refugees escaped to Venetian-held territories such as Methoni and Koroni, the Morea was slowly subdued, the last resistance being led by Constantine Graitzas Palaiologos, a relative of Thomas and Demetrios, at Salmenikon in July 1461.

Once it became clear that the Ottomans were marching towards Leontari and would soon arrive outside Mantineia,[33] Thomas, his entourage (including other Greek nobles, such as George Sphrantzes), his wife Catherine and his children Andreas, Manuel and Zoe fled to Methoni.

[36] Upon arriving in Rome, Thomas met with Pius II, who bestowed him with the Golden Rose, lodging in the Ospedale di Santo Spirito in Sassia and a pension of 300 ducats each month (for a total of 3600 annually).

[38] During his stay in Rome, Thomas, on account of his "tall and handsome appearance", served as the model of the statue of Saint Paul which to this day stands in front of the St. Peter's Basilica.

On 12 April 1462, Thomas gave the supposed skull of Saint Andrew the Apostle, a precious relic which had been in Byzantine hands for centuries, to Pius II.

Pius II had made the recovery of Constantinople one of the primary goals of his pontificate and his 1459 council at Mantua had secured the promise of an army amounting to a total of 80,000 men from various of the great powers in Western Europe.

[38] Thomas brought with him letters by Pius II who described him as "a prince who was born to the illustrious and ancient family of the Palaiologoi ... a man who is now an immigrant, naked, robbed of everything except his lineage".

[38] Like his father Manuel II and his brother John VIII before him, Thomas's possessed a certain royal charisma and good looks, which ensured that his appeals did not fall on deaf ears.

[43] Though Thomas had been largely bypassed and forgotten by the Roman elite after Pius II's death in 1464,[38] he was buried with honor in the St. Peter's Basilica,[37][43] where his grave would survive the destruction and removal of the tombs of the Palaiologan emperors in Constantinople during the early years of Ottoman rule.

[54] In the late 16th century, a family with the last name Paleologus, living in Pesaro in Italy, claimed descent from Thomas through a supposed third son, called John.