Parliamentary oversight of the military constitutes an important element of the civilian control of the armed forces. However, the strength of parliaments in this realm varies greatly across democracies and little is known about the sources of this variation. We propose an explanation for one key aspect of this variation: why does parliament enjoy veto power over military deployments in some democracies but not in others? Our analysis of data from forty-nine democracies around the world suggests that at least three factors account for parliamentary strength or weakness in this realm: the external threat to which a country is exposed, its constitutional tradition, and the experience of severe military failure in the past.