>>coldest summer on record in Europe, dating back to 1700

This was not the first time volcanism caused a climate cool down. There's some record (Procopius) of it happening in 536, then further cooling in 539–540. Justinian was emperor in Eastern Rome from 527-565 and this* might be why he (or Belisarius who was in his service) was un able to re-unify Eastern and Western empires

*plus an eruption of the plague, which may be related as starving people have poor immune systems.