Tom Perriello is a former member of Congress and conflict analyst who has conducted research in Afghanistan, Sierra Leone, and Darfur.

The use of force always entails grave dangers and human costs, and progressives have been leery particularly since the Vietnam era of supporting it, even to prevent or end mass atrocities, repression, and other systematic human suffering. Wise leaders will always remain wary of war. But wisdom also requires us to acknowledge two dramatic changes in our ability to use force for good. First, in a single generation, our ability to intervene without heavy casualties has improved dramatically. Second, the range of diplomatic and legal tools for legitimizing such interventions has likewise expanded. During this same period, we have been reminded tragically of the real and staggering human cost of inaction, most notably in the 800,000 lives lost in Rwanda. The tendency to feel less moral responsibility for the results of inaction and to overvalue the risks of acting in difficult situations is natural, but it is ultimately indefensible.

These new conditions present progressives today with a historic opportunity—to embrace a slight tipping of the scales toward action in the age-old balance between the horrors of the world and the horrors resulting from the military actions that might prevent them. This shift should be seen more as a marginal adjustment than as a dramatic ideological recalibration, but this new-generation understanding can mean the difference between paralysis and action.

Consider the post-Cold War era in American foreign policy. Putting aside for a moment the responses by George W. Bush’s Administration to September 11, this era offers three major examples of the use of American hard power since the fall of the Berlin Wall: under George H.W. Bush, the Persian Gulf War in response to a dictatorial invasion of a sovereign nation; under Bill Clinton, the Kosovo air campaign to stop ethnic cleansing; and under Barack Obama, the international campaign to oust Moammar Gadhafi from power and prevent attacks on civilians in Libya.

Whether or not one agrees with any or all of these missions, they share significant characteristics: a casus belli that mingled idealistic and realpolitik concerns; a cool (rather than hot) decision to proceed to war; the careful employment of regional and allied support; and the use of targeted, decisive force. It is not surprising in retrospect that Barack Obama claims a strong affinity for the foreign policy of the first President Bush. Seen as a continuum, these interventions represent the arrival of a new era of decisive pragmatism in the threat and use of American force, one in which the U.S. government has greater technological and normative capacity to act, and a growing body of case studies from which to refine its operational decisions to maximize its effectiveness...