Armed interventions levy enormous burdens, not least upon those with the capacity and will to mount them. When U.S. Secretary of State Colin Powell, referring to Iraq, opined to the president – “If we break it, sir, we will own it” – he was voicing a practical reality: interveners face far greater pressure today than in earlier eras to help stabilize and rebuild war-torn countries after major combat ends.

As a consequence, decisions to commit forces and resources to foreign interventions are among the most difficult ones that national leaderships face, especially in democratic countries. Governments must be convinced that contributing troops and other personnel serves their national interests and values, that the intervention has a reasonable prospect of success, that lesser strategies – for example, containment – cannot deliver the desired results, and in a larger sense that the benefits of the action outweigh its costs and risks. Recent experience in Iraq and elsewhere has also shown that states are far more likely to participate in an intervention – and contribute to post-conflict reconstruction – if they view the underlying intervention itself as legitimate.

The perceived legitimacy of an intervention will turn on many factors, including how urgent and compelling the circumstances are in the target state. Are terrorist groups using the territory to train operatives and launch attacks on other countries, with the support of the local regime? Are state actors or nonstate groups perpetrating horrific atrocities, in violation of fundamental norms of international law?