The assassination of Julius Caesar in 44 BCE triggered a power struggle that ultimately endedthe Roman Republic and, eventually, the Ptolemaic Kingdom, leading to the rise of the RomanEmpire. Climate proxies and written documents indicate that this struggle occurred during aperiod of unusually inclement weather, famine, and disease in the Mediterranean region;historians have previously speculated that a large volcanic eruption of unknown origin wasthe most likely cause. Here we show using well-dated volcanic fallout records in six Arctic icecores that one of the largest volcanic eruptions of the past 2,500 years occurred in early 43BCE, with distinct geochemistry of tephra deposited during the event identifying the Okmokvolcano in Alaska as the source. Climate proxy records show that 43 and 42 BCE were among the coldest years of recent millennia in the Northern Hemisphere at the start of one of thecoldest decades. Earth system modeling suggests that radiative forcing from this massive,high-latitude eruption led to pronounced changes in hydroclimate, including seasonaltemperatures in specific Mediterranean regions as much as 7oC below normal during the twoyear period following the eruption, and unusually wet conditions. While it is difficult toestablish direct causal linkages to thinly documented historical events, the wet and very coldconditions from this massive eruption on the opposite side of Earth probably resulted in cropfailures, famine, and disease, exacerbating social unrest and contributing to politicalrealignments throughout the Mediterranean region at this critical juncture of Westerncivilization.