The state represented by Peter I, breaking boyar Russia to the ground in order to build a new society, faced the phenomenon of the Cossacks, which was something third, special, did not belong to either the new or the old, but extremely valuable militarily. For this reason, the reformer tsar indicated that the Cossacks should unconditionally, voluntarily or forcibly, enter the general "regular" hierarchy. The era of Peter is a turning point in the history of Russia, and the "free" Cossack communities, and the Cossacks as a whole. This study comprehensively and comprehensively examined all aspects of the life of the Cossacks - religious, socio-political and military, the problems of their relationship with the authorities. For the first time, the real participation of the Cossacks in the Peter's Wars, especially in the Northern War, was studied and presented.