II, already in 1459, but both works were written decades after the events. Tursun Beg (a secretary in the sultan's court) stated that Vlad only turned against the Ottoman Empire when the sultan "was away on the long expedition in Trebizon" in 1461. According to Tursun Beg, Vlad started new negotiations with Matthias Corvinus, but the sultan was soon informed by his spies. Mehmed sent his envoy, the Greek Thomas Katabolinos (also known as Yunus bey), to Wallachia, ordering Vlad to come to Constantinople. He also sent secret instructions to Hamza, bey of Nicopolis, to capture Vlad after he crossed the Danube. Vlad found out the sultan's "deceit and trickery", captured Hamza and Katabolinos, and had them executed. After the execution of the Ottoman officials, Vlad gave orders in fluent Turkish to the commander of the fortress of Giurgiu to open the gates, enabling the Wallachian soldiers to break into the fortress and capture it. He invaded the Ottoman Empire, devastating the villages along the Danube. He informed Matthias Corvinus about the military action in a letter on 11February 1462. He stated that more than "23,884 Turks and Bulgarians" had been killed at his order during the campaign. He sought military assistance from Corvinus, declaring that he had broken the peace with the sultan "for the honor" of the king and the Holy Crown of Hungary and "for the preservation of Christianity and the strengthening of the Catholic faith". The relationship between Moldavia and Wallachia had become tense by 1462, according to a letter of the Genoese governor of Kaffa. Having learnt of Vlad's invasion, Mehmed II raised an army of more than 150,000 strong that was said to be "second in size only to the one" that occupied Constantinople in 1453, according to Chalkokondyles. The size of the army suggests that the sultan wanted to occupy Wallachia, according to a number of historians (including Franz Babinger, Radu Florescu, and Nicolae Stoicescu). On the other hand, Mehmed had granted Wallachia to Vlad's brother, Radu, before the invasion of Wallachia, showing that the sultan's principal purpose was only the change of the ruler of Wallachia. The Ottoman fleet landed at Brăila (which was the only Wallachian port on the Danube) in May. The main Ottoman army crossed the Danube under the command of the sultan at Nikopol, Bulgaria on 4June 1462. Outnumbered by the enemy, Vlad adopted a scorched earth policy