UNLABELLED: Despite continuous investigation, reasons for both the abandonment of Roman Britain around 410 CE, and the separate collapse of the Western Roman Empire in 476 CE remain unclear. Here, we use tree ring-based climate reconstructions and written documentary sources to show that a sequence of severe summer droughts from 364 to 366 CE not only contributed to prolonged harvest failures and food shortages, but also played a role in the 'Barbarian Conspiracy', a catastrophic military defeat for Roman Britain in 367 CE. In line with contemporary reports from the historian Ammianus Marcellinus, this pivotal event in pre-modern history coincided with anomalous coin hoarding, and a gradual depopulation of Roman villas and towns. Expanding our climate-conflict analysis from Roman Britain as a case study to the entire Roman Empire and the period 350-476 CE reveals clear linkages between years in which battles occurred and preceding warm and dry summers. Based on these findings, we develop a mechanistic model to explain the vulnerability of agrarian societies to climate variability, whereby prolonged droughts cause harvest failures and food shortages (dependant on societal resilience) that lead to systematic pressure, societal instability, and eventually outright conflict. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10584-025-03925-4.

Department of Geography Faculty of Science Masaryk University Brno 611 37 Czech Republic

Department of Geography University of Cambridge Cambridge CB2 3EN UK

Generaldirektion Kulturelles Erbe Rheinland Pfalz Direktion Rheinisches Landesmuseum Trier 54290 Trier Germany

Global Change Research Institute Czech Academy of Sciences Brno 603 00 Czech Republic

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