Steppe productivity decline contributed to smallpox outbreak and the collapse of the last nomadic empire in the 1750s.

The collapse of the Dzungar Khanate in the 1750s marked the end of the world's last nomadic empire and reshaped Eurasian geopolitics. However, whether environmental and societal factors interplayed and contributed to its collapse remains unclear. Here, we reconstructed steppe productivity variability across the Dzungar heartland since 1625 using 441 tree-ring series based on an extreme growth ratio. Our record shows that the Khanate's collapse coincided with the lowest steppe productivity in the past four centuries during 1751-1761. The weakening societal resilience approaching a tipping point intensified the effects of the rapid steppe productivity decline, which cascaded into famine, conflict, and trade breakdown. Smallpox then reemerged and escalated into a catastrophic epidemic, transforming the environmental crisis into the collapse of the Dzungar Khanate. Our findings underscore the urgent need to strengthen pastoral resilience and epidemic surveillance in steppe regions as climate stress intensifies and cross-border epidemic threats escalate.