Previous studies of internal armed conflict outcomes have found evidence that rebel-biased military intervention increases the likelihood of rebel victory, but little indication that pro-government interventions improve the odds of government victory. Our argument, grounded in a theory of the utility and limitations of military force in civil wars, anticipates that armed intervention increases the probability of victory for the supported side only when that belligerent’s primary challenge is a lack of conventional war-fighting capacity. Empirical analyses of internal armed conflicts from 1945 to 2010 support these expectations. Direct interventions in support of opposition movements have substantively large, robust effects on conflict outcomes. In contrast, government-biased interventions are only effective in increasing the odds of an outcome favorable to the government when the fighting capacity of rebel forces matches or exceeds that of the state.