Europe to act in the defense of his collapsing empire, threatened by the rapidly expanding Ottoman Empire. John left Constantine, his most trusted brother, and favored heir, as regent in Constantinople. Demetrios, alongside many distinguished Byzantine nobles and church officials (including the Patriarch of Constantinople, Joseph II) accompanied John on his journey to Italy. Although Demetrios was known to be firmly against any plans of reuniting the Orthodox and Catholic churches, he had already shown rebellious tendencies, hoping to become emperor himself with Ottoman support, and could not be trusted to remain in Constantinople or Lemnos when the emperor was away. While the Council of Florence did achieve a union of the churches, it lasted for several years, concluding in June 1439. John didn't return to Constantinople until 1 February 1440 and though he was received with a grand and pomp ceremony, organized by Constantine and Demetrios (who had returned sometime earlier), the news that the churches had been unified stirred a wave of resentment and bitterness among the general populace, who felt that John had betrayed their faith. Attempts to become emperor With his tenure as regent finished, Constantine had observed that the other Despots of the Morea (Constantine's former appanage), his brothers Theodore and Thomas, had ruled well without him and he considered that he could serve the empire's interest better if he was closer to the capital. Demetrios now governed a stretch of land around the city Mesembria in Thrace and Constantine considered the idea that he and Demetrios could switch places, Constantine becoming the new governor in Mesembria and Demetrios being granted the Morea. Constantine thus sent his close advisor George Sphrantzes to forward the idea to both Demetrios and Ottoman Sultan Murad II, who by this point had to be consulted about any appointments. By 1442, Demetrios had no desire for new appointments, instead eyeing the imperial throne. He 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. When Sphrantzes reached Demetrios to forward Constantine's offer, Demetrios was already preparing to march on Constantinople. The danger he posed to the city was so great that Constantine was summoned from the Morea by John VIII to oversee the city's