By the 15th century, the Byzantine emperors had lost any real power, with the Ottoman sultans increasingly becoming the real regulators of political matters within the empire. Despite their "de facto" submission under the Ottomans, the Byzantines continued to act with hostility against them. In 1394, Manuel II ceased to pay tribute to the Ottoman sultan Bayezid I, who responded by besieging Constantinople. Bayezid's siege lasted for more than a decade, with Western aid through the Crusade of Nicopolis (1396) failing to stop the sultan. The situation was so dire that Manuel left Constantinople to travel around Western Europe asking for further aid against the Ottomans, visiting Italy, France and England. Ultimately, the siege was resolved peacefully by Manuel through signing a peace treaty with Bayezid's son and successor, Mehmed I, in 1403, which among other things returned the city of Thessaloniki, captured by the Ottomans in 1387, to Byzantine rule.