There's been some research in that questoin, several of which have demonstrated the relationship between climate, the ability to produce food, and warfare.
One study, looking at the pacific oscillation studies, perhaps put it best:
"Precipitation, temperature, sunlight, humidity and ecological extremes can adversely influence both agrarian and non-agrarian economies. In addition, ENSO variations affect natural disasters, such as tropical cyclones, and trigger disease outbreaks. All of these have adverse economic effects, such as loss of income or increasing food prices, and it is thought that economic shocks can generate civil conflict through a variety of pathways. Furthermore, altered environmental conditions stress the human psyche, sometimes leading to aggressive behaviour. We hypothesize that El Niño can simultaneously lead to any of these adverse economic and psychological effects, increasing the likelihood of conflict."
http://junksciencecom.files.wordpres...-war-study.pdf
Studies which like the unusually strong variations in climate to the eventual and protracted end of the Roman Empire:
" Wet and warm summers occurred during periods of Roman and medieval prosperity. Increased climate variability from ~AD 250 to 600 coincided with the demise of the Western Roman Empire and the turmoil of the Migration Period"
And later: "Around 1300 C.E., on the other hand, a cold snap combined with wetter summers coincides with widespread famines and plague that wiped out nearly half of Europe's population by 1347. " And since you asked about "wars," the huge disruption in food production being one of the underlying causes of the Hundred Years War.
2500 Years of European Climate Variability and Human Susceptibility
In another study researchers examined more recent changes over the past few hundred years, food prices, population and number of wars.
"Further analyses show that cooling impeded agricultural production, which brought about a series of serious social problems, including price inflation, then successively war outbreak, famine, and population decline successively. The findings suggest that worldwide and synchronistic war–peace, population, and price cycles in recent centuries have been driven mainly by long-term climate change. The findings also imply that social mechanisms that might mitigate the impact of climate change were not significantly effective during the study period. Climate change may thus have played a more important role and imposed a wider ranging effect on human civilization than has so far been suggested."
Global climate change, war, and population decline in recent human history
Another good example shows the impacts on sensitive areas, such as probably ended the Anasazi peoples.
" The periods of wetness and dryness they identified coincided with dramatic changes in the Anasazi's way of life."
Climate Changes Coincide with Cultural Shifts in Ancient American Southwest: Scientific American
There's a pretty strong case that the very long droughts in the horn of Africa have contributed to the violence, famines and and establishment of terror organizations willing to exploit the situation over the past few decades.
Pentagon think tanks have released two reports in the past ten years that in their view listed climate change as the number one destabilizing cause that would lead to warfare and risk to US National security. (both were mostly ignored by Congress and the White house).