According to our Gemara, the party that opens Megillat Esther was thrown by Ahashverosh to celebrate the fact that 70 years had passed since the Temple was destroyed and the Jews exiled, yet the Temple had not been rebuilt. The tradition of the Sages was that the Babylonian and Persian kings were well aware of Yirmiyahu’s prophecy that the first exile would last 70 years (see Yirmiyahu 29:10), and the party described in Sefer Daniel (see Chapter 5) – where King Belshazzar brought out the Temple vessels that had been looted by Nevuhadnezzar – was celebrating the fact that the Jews had not returned to their land and that the prophecy had not been fulfilled. That party ended in disaster, with Daniel reading the proverbial “handwriting on the wall” that foretold King Belshazzar’s death. Ahashverosh was convinced that Belshazzar had been mistaken in counting the 70 years from the beginning of the Babylonian empire, but that he could now celebrate, since it was 70 years since the beginning of the Jewish exile under King Yehoyakhin (see II 24:8-16).
The Ramban explains that Ahashverosh did not reject the prophecy entirely. He felt that the permission given by King Koresh (Cyrus) to the Jews to return to Israel was sufficient for the prophecy to be considered fulfilled, but that the Temple would not be rebuilt. Some suggest that his acceptance of the prophecy is what allowed him to live, even as Belshazzar was killed.
The Ramban also explains that it was the Temple vessels that had been looted when King Yehoyakhin was taken into exile that were used in King Belshazzar’s party. Those same vessels were returned to the Jewish community by King Koresh when he allowed them to return to their land. There were, however, other vessels that had been looted when King Tzidkiyahu was exiled (see II 25:8-17), and those vessels were used by Ahashverosh at his party. These vessels were eventually returned to the Jews, as well, when King Artahshasta (Artaxerxes) encouraged Ezra ha-Sofer to lead the Jews back to Israel and build the Second Temple (see Ezra 7:19).