Around the end of the 16th century, increasing Cossack aggression strained relations between the Commonwealth and the Ottoman Empire. Cossacks had begun raiding Ottoman territories in the second part of the 16th century. The Polish government could not control them, but was held responsible as the men were nominally its subjects. In retaliation, Tatars living under Ottoman rule launched raids into the Commonwealth, mostly in the southeast territories. Cossack pirates responded by raiding wealthy trading port-cities in the heart of the Ottoman Empire, as these were just two days away by boat from the mouth of the Dnieper river. In 1615 and 1625, Cossacks razed suburbs of Constantinople, forcing the Ottoman Sultan to flee his palace. In 1637, the Zaporozhian Cossacks, joined by the Don Cossacks, captured the strategic Ottoman fortress of Azov, which guarded the Don.