It must be exclusive, no third-parties
James D. Morrow 2K, PhD, Professor, Poli Sci, Stanford University, June 2000, "Alliances: Why Write Them Down?", Annual Review of Political Science, Vol. 3, https://www.annualreviews.org/doi/10.1146/annurev.polisci.3.1.63
Whether external or internal audiences impose costs on leaders who fail to honor their states' commitments, audience costs can operate as a commitment device. Because of these costs, the likelihood for a state to intervene on behalf of an ally is greater than it would have been for the same state to intervene in the same crisis without the alliance. The formal alliance thus serves as a commitment device. Without it, the relevant audience would not hold the leader or state responsible for its failure to intervene on behalf of the state whose interests it shares. With it, a state's leader knows that she may face serious consequences if she backs out of her commitment. States and their leaders thus make alliance commitments only to those states whose interests they share and only if they are confident that they will live up to their commitment to their allies in the event of war. Because the precise issues of a crisis and the positions of the contending sides cannot be predicted, it is possible that a state may choose not to intervene despite an alliance and its commitment effects.