Frightened of the Jews – Purim
In the Purim story there is another group who did the opposite. Called Mityahadim, they tried to make out that they were not gentiles but Jews. In Esther 8:17 they acted like Jews; Rashi says they became converts, though Maimonides – Issurei Bi’ah 13:15 – says they remained gentiles but gave the impression of Jewishness.
Their motivation was self-preservation; the Jews had assumed a leading role in the kingdom and it was expedient to be on the Jewish side and to have the Jewish God.