Three years ago, I went to a Shiur at the ORTHODOX UNION in Jerusalem where Rabbi Chaim Eisen spoke. The Rabbi explained further reasons on why we commemorate the 10th of Tevet.
It is said that the great Prophet Ezra who took thousands of Jews out of the 70 – year lasting Babylonian Diaspora back to Eretz Israel, died on 9th Tevet. The passing of Ezra was a big loss for the Jewish nation. Still back in Babylon, Ezra had thought that all Jews would speedily return to Israel as soon as the Persian King gave his permission. However, Ezra was wrong because tons of Jews didn’t even consider a return to Israel, as they had built themselves a very convenient life in the Diaspora. Israel ? Jerusalem ? Temple ? Everything is destroyed. Who needs that ?
Ezra wrote to many communities and his proposal of returning to the Holy Land got rejected by many Jews. The same happened with the German communities Speyer, Worms and Mainz. After the destruction of the First Temple, Jews had already settled in Germany.
However, the Jews from those three German communities refused to return to Eretz Israel and restore the Temple. Not only them but also them. And it happened particularly to those three communities which were almost destroyed by the Crusaders on their first Crusade in 1096. The Crusaders passed those towns and killed more than 100,000 Jews.
The Talmud teaches us that from hundreds of thousands of Jews living in the Babylonian Diaspora, only 42,360 returned to Eretz Israel. The rest remained in Babylonia. Therefore, the Second Temple lost parts of its holiness compared to the First Temple. During the First Temple period, the Shechinah (G – d’s presence) could be felt and was always around. The Ark was still standing in the Holy of Holies (Kodesh HaKedoshim), the High Priest (Cohen HaGadol) wore the Urim ve’Turim (Breastplate), G – d’s secret names were still known and also their pronunciation and Prophets were around everywhere. If all the Jews had returned from the Diaspora and build up the Second Temple, G – d may have granted them much more than a limited holiness of the rebuild Temple.
On Asarah be’Tevet, we don’t only commemorate the siege of Jerusalem but the times of missed opportunities. If the Jews had reacted in different ways, the future and our present may have looked very different. Who of the Diaspora Jews thinks about Jerusalem at all ? Who constantly desires a Third Temple ? Most Jews say their prayers including the prayer for the rebuilding of Jerusalem but, as soon as the prayer is over, everyone goes back to normal.
Thus, we mourn and fast because of the missed opportunities and the behaviour of the Jews. When Jews are getting too comfortable in the Diaspora, they act against G – d’s will.