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Moisture conditions, especially those that occur as multi-decadal anomalies, have profound impacts on society, especially in arid and semi-arid regions. However, the lack of high-resolution climate data for the first millennium CE greatly limits our understanding of how moisture variations have influenced history. Here, we present an 1882-year (134–2015 CE) tree-ring chronology developed from Qilian juniper (Juniperus przewalskii Kom.) growing in the western Qilian Mountains, northwest China. The tree-ring index correlates significantly with the May–June self-calibrating Palmer Drought Severity Index (sc-PDSI) and can be used to reconstruct May–June moisture variations since 241 CE. The reconstruction reflects moisture conditions at the annual to multi-decadal time scales over the past two millennia. During the period from the third to eighth centuries, there were prominent interdecadal fluctuations, with the third century and the late fifth century being the wettest and driest periods during the reconstruction, respectively. The transition from the wet third century to the dry fifth century corresponded with key events in Chinese history, namely the demise of the Western Jin Dynasty and the chaotic Southern and Northern Dynasties. Thus, our reconstruction provides new evidence for the potential linkage between abnormal climate conditions and social changes in ancient times. [ABSTRACT FROM AUTHOR]