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Extreme weather events and military conflict over seven centuries in ancient Korea

Significance We examine the link between extreme weather events and interstate conflict using data covering the three ancient kingdoms on the Korean Peninsula over seven centuries. Extreme weather events, such as drought and heavy rainfall, have a direct causal effect on conflict, but this effect is asymmetric: Shocks are far more likely to give rise to opportunistic invasions (where the state experiencing the shock is attacked) than to desperate invasions (where the state experiencing the shock initiates attack). We identify food insecurity as a critical source of vulnerability. Since climate change is leading to increased frequency of extreme weather events, a better understanding of the implications for labor-intensive armed conflict can help with mitigation strategies. We explore the causal connection between weather and war by constructing and analyzing a dataset featuring extreme weather events and military conflicts involving a set of stable political entities that existed side by side over several centuries, namely, the three ancient kingdoms of the Korean Peninsula between 18 Before the Common Era and 660 Common Era. Conflicts are classified as desperate if a state experiencing the shock invades a neighbor and opportunistic if a state experiencing the shock is invaded by a neighbor. We find that weather-induced conflict was significant, but largely opportunistic rather than desperate. That is, states experiencing an adverse shock were more likely to be invaded, but not more likely to initiate attack. We also provide evidence that the channel through which weather shocks gave rise to opportunistic invasions was food insecurity, which weakened the power of states to repel attack. Since climate change is projected to give rise to an increased frequency of extreme weather events, these historical findings have contemporary relevance.