Under what conditions do communities in ethnic wars form, and abide by, local non-aggression pacts? Studies of civilian victimization showed that pre-existing microlevel contention can generate violence, but lesser attention was given to the role of cordial relationships in constraining it. In this paper I argue that local actors will create inter-ethnic agreements in order to coordinate when there is high uncertainty over the local outcome of the war, and thus cooperatively lower it by providing mutual assurances. They will then abide by their commitment for mutual protection only while the uncertainty persists, since once one side knows it has the upper hand, the alliance will not be needed, and will be broken. I test my expectations using a variety of original quantitative and qualitative evidence from the First Jewish-Arab War (1947-9). The results strongly support my theory even when various statistical models and methods of causal inference are used and cases are examined in more focus.