Byzantine Emperor Heraclius launches a massacre of Jews, killing tens of thousands in Jerusalem and throughout Galilee. One explanation, at least according to Eutychius, is that Heraclius is encouraged to to this by antisemitic priests or monks.
Another explanation is that Heraclius had a dream that the Byzantine Empire would be destroyed by a "circumcised people." Heraclius took this to be the Jews and so had them slaughtered to protect himself. Later, though, the Byzantine Empire is destroyed by another circumcised people that had recently arisen in Arabia: Islam.