Khmelnytsky's initial successes were followed by a series of setbacks as neither Khmelnytsky nor the Commonwealth had enough strength to stabilise the situation or to inflict a defeat on the enemy. What followed was a period of intermittent warfare and several peace treaties, which were seldom upheld. From spring 1649 onward, the situation turned for the worse for the Cossacks; as Polish attacks increased in frequency, they became more successful. The resulting Treaty of Zboriv on 18 August 1649 was unfavourable for the Cossacks. It was followed by another defeat at the battle of Berestechko on 18 June 1651 in which the Tatars betrayed Khmelnytsky and held the hetman captive. The Cossacks suffered a crushing defeat, with an estimated 30,000 casualties. They were forced to sign the Treaty of Bila Tserkva, which favoured the Polish-Lithuanian Commonwealth. Warfare broke open again and, in the years that followed, the two sides were almost perpetually at war. Now, the Crimean Tatars played a decisive role and did not allow either side to prevail. It was in their interests to keep both Ukraine and the Polish–Lithuanian Commonwealth from getting too strong and becoming an effective power in the region.