stylistic:style_transfer, structural:reformatting, structural:summarization
Initial successes

At the end of 1647 Khmelnytsky reached the estuary of the Dnieper river. On 7 December, his small detachment (300–500 men), with the help of registered Cossacks who went over to his side, disarmed the small Polish detachment guarding the area and took over the Zaporozhian Sich. The Poles attempted to retake the Sich but were decisively defeated as more registered Cossacks joined the forces. At the end of January 1648, a Cossack Rada was called and Khmelnytsky was unanimously elected a hetman. A period of feverish activity followed. Cossacks were sent with hetman's letters to many regions of Ukraine calling on Cossacks and Orthodox peasants to join the rebellion, Khortytsia was fortified, efforts were made to acquire and make weapons and ammunition, and emissaries were sent to the Khan of Crimea, İslâm III Giray.

Initially, Polish authorities took the news of Khmelnytsky's arrival at the Sich and reports about the rebellion lightly. The two sides exchanged lists of demands: the Poles asked the Cossacks to surrender the mutinous leader and disband, while Khmelnytsky and the Rada demanded that the Commonwealth restore the Cossacks' ancient rights, stop the advance of the Ukrainian Greek Catholic Church, yield the right to appoint Orthodox leaders of the Sich and of the Registered Cossack regiments, and to remove Commonwealth troops from Ukraine. The Polish magnates considered the demands an affront, and an army headed by Stefan Potocki moved in the direction of the Sich.

Had the Cossacks stayed at Khortytsia, they might have been defeated, as in many other rebellions. However, Khmelnytsky marched against the Poles. The two armies met on 16 May 1648 at Zhovti Vody, where, aided by the Tatars of Tugay Bey, the Cossacks inflicted their first crushing defeat on the Commonwealth. It was repeated soon afterwards, with the same success, at the Battle of Korsuń on 26 May 1648. Khmelnytsky used his diplomatic and military skills: under his leadership, the Cossack army moved to battle positions following his plans, Cossacks were proactive and decisive in their manoeuvrers and attacks, and most importantly, he gained the support of both large contingents of registered Cossacks and the Crimean Khan, his crucial ally for the many battles to come.