On the 13th of Adar the Jews gathered to defend their lives and take vengeance on their enemies. They had to plead to Hashem for mercy that he come to their aid.
- When the yidden went to battle they fasted so Hashem should help them.
- Moshe fasted on the day they went to battle with amalek
We assume the 13th of Adar the yidden fasted as well before the battle and we fast the day before purim too. We call it Tannis Esther, The Fast of Esther.
