The main questions explored here are whether alliances lead to conflict between member states and non-member states and whether the capability of allies is a source of this effect. Building on the opportunity framework, this study argues that, with more confidence in military success due to support from allies, challenger states are more likely to be emboldened and to initiate disputes. The empirical analyses show that the capability of allies has an increasing effect on dispute initiation of member states against non-member states. This increasing effect becomes stronger as the level of common interests between the allies increases. The test results consistently suggest that alliances embolden member states to initiate disputes against non-member states, and also that alliances are more likely to aggravate, but not mute, dispute initiation against member states.