Земский собор
The conflict was triggered by the Khmelnytsky Rebellion of Ukrainian Cossacks against the Polish–Lithuanian Commonwealth. The Cossack leader, Bohdan Khmelnytsky, derived his main foreign support from Alexis of Russia and promised his allegiance in recompense. Although the Zemsky Sobor of 1651 was poised to accept the Cossacks into the Moscow sphere of influence and to enter the war against Poland on their side, the Tsar waited until 1653, when a new popular assembly eventually authorized the unification of Ukraine with Tsardom of Russia. After the Cossacks ratified this agreement at the Pereyaslav Rada the Russo-Polish War became inevitable.