I investigate the determinants of interstate political alignment, examining why states take part in ongoing conflicts and which side they take in them. The puzzle I seek to address is why some states are much more likely to gain support than others, and whether the likelihood of such support varies on the basis of the issue under dispute and the characteristics of the state itself. I emphasize the interests of rulers, particularly their need to obtain support on issues of high salience to them. The desire for future reciprocity lies at the heart of these alignment decisions. First, leaders consistently reciprocate positive and negative alignments. Second, rulers avoid positively aligning with leaders of unstable or politically unrepresentative states, as the latter are less likely to be in a position to return the favor. In order to test this alignment explanation, I compile a dataset of interventions into existing wars, MIDs, and sanctions regimes, covering the 1816–1999 time period. The results show that not all types of states are likely to enter an ongoing conflict. When those states do join a dispute, they do so on the side of those who helped them in the past.