The article deals with the motivation and consequences of the Terek Cossacks’ defection to the side of the opponents of the tsarist military administration in the North Caucasus. The author makes an attempt to analyze the motivation of the Cossacks’ defection and their subsequent military activities on the side of the mountain rebels. The presence of Shamil in the troops, as well as of other mountain leaders of the Caucasian War era of a considerable number of the Terek Cossacks indicates the complexity and ambiguity of the participation of the Cossacks and the North Caucasus representatives in the Caucasian War and the failure of their unambiguous opposition in the considered period.