Drought facilitated the westward expansion of the Mongol Empire in the 1230s.

The Mongol Empire's Western Expedition stands as the most impactful historical event shaping the historical trajectory of the Eurasian continent in the second millennium. However, the climatic spatiotemporal patterns during the Mongol Empire's Western Expedition have remained inadequately elucidated. Here, we reconstruct changes in summer water balance on the East European Plain between 943 and 2019 CE. Integrating tree-ring hydroclimatic sequences from Central Europe, Central Asia, and East Asia, we comprehensively delineate the climatic backdrop along the empire's western expansion route, demonstrating the pivotal role of climate in driving the Mongol Empire's expansion. We find that the persistent arid grassland environment along the main routes of the Mongol Empire's westward expansion (1227-1241 CE) provided climatic advantages for their expansion, enhancing the military effectiveness of Mongol cavalry and driving the rapid conquest of the Eurasian continent. We identify the summer North Atlantic Oscillation as a primary driver of East European Plain hydroclimatic variability and highlight increased drought risk under future warming scenarios. By integrating long-term climate variability with sociohistorical dynamics, this study proposes a framework for understanding the mechanisms underpinning climate-human interactions.