In March 2021, economics and human rights professor Rajiv Sethi published new research in the journal Proceedings of the National Academy of Sciences of the USA (PNAS) examining how extreme weather can impact military action. Using three ancient kingdoms on the Korean Peninsula as a case study, Sethi and his colleague observed a direct relationship between climate disasters and aggression, noting how countries experiencing "extreme weather events" and loss of resources are more likely to be invaded by their neighbors in "opportunistic" attacks. Sethi's historical findings can be likened to the effects of climate change on countries today, where food insecurity has made afflicted countries increasingly defenseless.