It makes sense doesn’t it, nations enjoying prosperity don’t like to make war, during hard times politicians need to keep things stirred up to avoid rebellion at home.
A great tool of despots everywhere is to create a distracting threat from outside.
Warm times are good times–less need to keep the populace angry at some foreign entity.
Also, warm periods/periods of plenty tend to mean less need to go take what you need to survive.
The time period including the fall of Rome saw brutal winters and shorter than normal growing seasons all over Europe. While the various “barbarian” tribes saw no reason to attack a hard target (Rome) during times of plenty, times of scarcity made well have made the risk seem well worth the potential gain. It may also have further weakened an already tottering empire.