t the end of the megillah Esther begs Achashveirosh to rescind the decree against Bnei Yisrael (8:5):
יִכָּתֵ֞ב לְהָשִׁ֣יב אֶת־הַסְּפָרִ֗ים מַחֲשֶׁ֜בֶת הָמָ֤ן בֶּֽן־הַמְּדָ֙תָא֙ הָאֲגָגִ֔יAchashveirish responds that he cannot do that, as once promulgated, the law cannot be rescinded. But what he can do is send another letter allowing the J to fight back (8:8):
Everyone is thrilled! Time to celebrate!
לַיְּהוּדִ֕ים הָֽיְתָ֥ה אוֹרָ֖ה וְשִׂמְחָ֑ה וְשָׂשֹׂ֖ן וִיקָֽר׃
R' Chaim Kanievsky asks: why were people already jumping for joy? They weren't out of the woods yet! They would have to fight a war against the Aggagi - Amaleiki. According to some Rishonim the reason we have taanis Esther is because that was the day we gathered for battle. People's lives were still on the line.
R' Chaim answers (Taama d'Kra) that this was a deliberate ploy by Mordechai. If you act like a winner, people think you are a winner. People treat you like a winner. Mordechai said to act like the battle was won, like they held all the cards now. It may not have been true, but the point is that it gave the impression that it was true. רַבִּ֞ים מֵֽעַמֵּ֤י הָאָ֨רֶץ֙ מִֽתְיַֽהֲדִ֔ים כִּֽי־נָפַ֥ל פַּֽחַד־הַיְּהוּדִ֖ים עֲלֵיהֶֽם Psychologically, the tide had turned. One that happens, the battle is in the bag.
A couple of years ago I quoted the SHL"h's interpretation of the midrash
תשורי מראש אמנה
עתידין ישראל לומר שירה לעתיד לבוא, שנאמר: (תהלים צח, א)שירו לה' שיר חדש כי נפלאות עשה.
What's the double language of עתידין ישראל לומר שירה לעתיד לבוא? If it is עתידין then it obviously is לעתיד לבוא? Says the SHL"H: when we reach the ultimate geulah, we won't wait for the miracles to all happen before we start praising Hashem. עתידין in the future we will sing shirah to Hashem לעתיד לבוא, on the miracles that we know and are confident are going to happen, even if they haven't occurred yet (there is a R' Chaim that is against this approach, but not everyone has to agree with R' Chaim.)
Maybe לַיְּהוּדִ֕ים הָֽיְתָ֥ה אוֹרָ֖ה וְשִׂמְחָ֑ה וְשָׂשֹׂ֖ן וִיקָֽר because at that moment we were on the level of celebrating in anticipation of the nes rather than as a reaction afterwards. Maybe it was that bitachon that the nes will happen, that we will be victorious, which itself is the driving force that brought victory to fruition.