A dynamic model of a game of two coalitions under uncertainty is constructed. As the solution, a coalition G-guaranteed equilibrium of threats and counterthreats is proposed. It is proposed that, in this equilibrium situation, one coalition has the possibility to win over a player of the other coalition into this coalition. The definitions of threats and counterthreats of coalitions for 'winning over' players are formulated. On the basis of these definitions, the notion of a stable coalition structure is introduced. Some sufficient conditions of the existence of threats and counterthreats in the game of 'winning over' and the stability of the cooperative structure are obtained.