All these things have happened in the past even when there was no warming period.Crop failures, drought, famine, epidemic disease, kingdoms collapsing, migrations, invasions pillaging across whole continents where we have recorded histories
But okay, lets say your theory is correct, can you then explain why the Roman Empire flourished during the Roman Warming period.
At one point the Romans had conquered almost all of Western Europe.
While you're at it could you also explain why that warming period happened??
https://en.wikipedia.org/wiki/Roman_Warm_Period