According to the author of the book of Ezra, some people wrote a mean letter to the king to complain about the people living in Judah and Jerusalem. They wanted to get the king to not like them and maybe stop them from building their city.
Ezra 4:6: And in the reign of Ahasuerus, in the beginning of his reign, wrote they [unto him] an accusation against the inhabitants of Judah and Jerusalem. [Ahasuerus: Heb. Ahashverosh]
Ezra 4:6 mentions a letter written during King Ahasuerus's reign to oppose the Jews. The contradiction comes from historical timing since Ahasuerus is often identified with Xerxes I, who ruled after the time traditionally associated with the events in Ezra. This creates confusion about the chronological order of events.