Last year marked the centennial anniversary of the infamous ‘Christmas Truce’ of 1914. Just five months into World War I, the fighting among European powers had already taken the tragic and deadly shape that we now popularly associate with it: trench warfare. Total casualties from the fighting (military and civilians combined) would climb into the tens of millions by war’s end. In the backdrop of what was considered to be a “war to end all wars,” it is particularly remarkable to reflect upon the events of Christmas Eve, 1914. Although evidence is incomplete, records indicate that German soldiers, while singing Christmas carols, slowly and with hands raised, approached British soldiers, who reciprocated by doing the same. Spontaneously, and at risk of treason for fraternization with the enemy, frontline soldiers were able to briefly look past their coalitional imperatives in order to, at the very least, enjoy a respite from the hell of war. They exchanged stories, gifts, allowed each side to recover fallen comrades, and allegedly even engaged in a game of soccer.This short idyllic interlude in an otherwise long and gruesome war helps to emphasize some obvious yet profound features of warfare. First, it is made possible by a deeply ingrained coalitional psychology. In Matt Ridley’s words, humans are inescapably ‘groupish,’ and social life is defined across most if not all dimensions in terms of group identities. Second, and seemingly as a by-product of these group affiliations, we seem to find it easy to derogate and dehumanize outgroups whose success or vitality seems to come at the expense of our own. The flip side of this intergroup competition is that it can have the by-product of enhancing good feelings about our own group, which can manifest as anything from a ritual dance to xenophobic nationalism. Third, however, humans display a nearly unsurpassed entrepreneurial ability to redraw and redefine the boundaries between coalitions. History is replete with examples of peaceful communities being torn overnight into civil war, and also with examples of enduring enemies uniting under shared goals or beliefs.Research on the evolution of war has begun to shed light on some of the more puzzling and enduring aspects of warfare. For example, researchers examining motivations for war have made gains in understanding the role of anger, hatred, revenge, and sacred values. This research suggests that humans possess psychological adaptations designed by natural selection that shape our beliefs and motivations between and within groups in the context of warfare. In parallel, research has begun to take seriously the human capacity for cooperation, forgiveness, and peacemaking. So where does this leave us and where are we going?As we deepen our understanding of the many aspects of warfare, we must simultaneously give attention to the dynamic relationships among these aspects. For example, we have improved our understanding of the conditions that precipitate hatred and revenge, and have a clearer understanding of the conditions under which forgiveness and reconciliation can occur. But how do we get from one to the other? How and when do warring societies lay down their arms and forgive? Why was a ‘Christmas Truce’ possible in the winter of 1914, one of the bloodiest wars in history, but such a truce would seem laughable in the context of international terrorism or insurgency? How do coalitional enemies come to see themselves as part of a larger whole? These challenges reflect a distinction economists often draw between statics and dynamics. Scientists have made great strides toward understanding the sets of conditions that precipitate certain outcomes (statics), but the next challenge is to explain movement from outcome to outcome over time (dynamics).This effort would be particularly helpful for bringing certain debates to an end that have long outlived their relevance, such as the question of whether we are ‘wired for war’ or ‘peacemaking primates.’ Instead, a dynamic approach to coalitional phenomena would ask about the interaction of psychological mechanisms for fighting and reconciliation, and reveal the processes by which a slide in either direction is likely. We have the evolved capacity for both fighting and reconciliation, and neither is inevitable. Just as the soldiers of the ‘Christmas Truce’ lived the experience of their psychological plasticity, modern behavioral scientists must give greater attention to the dynamic interaction of evolved mechanisms for war and peace, rather than studying each in exclusivity. Indeed, a truce between advocates of war in human evolution and advocates of peacemaking in human evolution can open the door to deeper understanding of the dynamic slide between war and peace, rather than falsely presupposing the timeless dominance of one over the other.