lasted until the 20th century. His second attempt to conquer Vienna failed in 1532, as Ottoman forces were delayed by the siege of Güns and failed to reach Vienna. In both cases, the Ottoman army was plagued by bad weather, forcing them to leave behind essential siege equipment, and was hobbled by overstretched supply lines. In 1533 the Treaty of Constantinople was signed by Ferdinand I, in which he acknowledged Ottoman suzerainty and recognised Suleiman as his “father and suzerain”, he also agreed to pay an annual tribute and accepted the Ottoman grand vizier as his brother and equal in rank. By the 1540s, a renewal of the conflict in Hungary presented Suleiman with the opportunity to avenge the defeat suffered at Vienna. In 1541, the Habsburgs attempted to lay siege to Buda but were repulsed, and more Habsburg fortresses were captured by the Ottomans in two consecutive campaigns in 1541 and 1544 as a result, Ferdinand and Charles were forced to conclude a humiliating five-year treaty with Suleiman. Ferdinand renounced his claim to the Kingdom of Hungary and was forced to pay a fixed yearly sum to the Sultan for the Hungarian lands he continued to control. Of more symbolic importance, the treaty referred to Charles V not as 'Emperor' but as the 'King of Spain', leading Suleiman to identify as the true 'Caesar'. In 1552, Suleiman's forces laid siege of Eger, located in the northern part of the Kingdom of Hungary, but the defenders led by István Dobó repelled the attacks and defended the Eger Castle. Ottoman–Safavid War Suleiman's father had made war with Persia a high priority. At first, Suleiman shifted attention to Europe and was content to contain Persia, which was preoccupied by its own enemies to its east. After Suleiman stabilized his European frontiers, he now turned his attention to Persia, the base for the rival Islamic faction of Shi'a. The Safavid dynasty became the main enemy after two episodes. First, Shah Tahmasp killed the Baghdad governor loyal to Suleiman, and put his own man in. Second, the governor of Bitlis had defected and sworn allegiance to the Safavids. As a result, in 1533, Suleiman ordered his Pargalı Ibrahim Pasha to lead an army into eastern Asia Minor where he retook Bitlis and occupied Tabriz without resistance. Suleiman joined Ibrahim in 1534. They made a push towards Persia, only to find the Shah sacrificing territory