The civil-military interface in peace support operations is changing due to increasingly overlapping tasks, increased military involvement in humanitarian activities, and increased integration of all involved actors, not least through various current strategic concepts. This article not only describes these trends, but also, more importantly, analyses certain consequences in terms of mission effectiveness. The focus of the analysis is the ideas of 'militarisation of humanitarian aid' and the reverse 'humanitarianisation of the military'. The main arguments of this contribution are that the assumptions of increased effectiveness stemming from civil-military integration cannot be taken for granted and that there are harmful consequences stemming from blurring the lines between civilian, humanitarian and military actors. There is, in other words, a need to better specify and explain the causal mechanisms that lead to effectiveness in complex peace support operations.