The Mishna Berura in תרפו, ב talks about how in the days of Mordechai and Esther the Jews must have fasted because it was a time when they needed mercy just like when the Jews went to war against Amaleik, Moshe fasted. My question is, I understand the regular Jews who weren't fighting fasted but did the soldiers fast? The soldiers needed energy to fight, so wouldn't it be Pikuach Nefesh to fast? Thanks in advance for any source based answers.
As it turns out, I think my father-in-law found the answer in שולחן ערוך אֹרח חיים, תקע״א (Shulchan Aruch, Orach Chaim, Taf Kuf Ayin Aleph)
עִיר שֶׁהִקִּיפוּהָ עַכּוּ''ם, וְכֵן יָחִיד הַנִּרְדָּף מִפְּנֵי עַכּוּ''ם אוֹ מִפְּנֵי רוּחַ רָעָה, אֵינָם רַשָּׁאִים לְהִתְעַנּוֹת, שֶׁלֹּא לְשַׁבֵּר כֹּחָם; אֶלָּא יְקַבְּלוּ עֲלֵיהֶם לְהִתְעַנּוֹת כָּךְ וְכָךְ תַּעֲנִיּוֹת לִכְשֶׁיִּנָּצְלוּ; וְכֵן אִם גָּזְרוּ גְּזֵרָה וְגָזְרוּ שֶׁלֹּא לְהִתְעַנּוֹת עָלָיו, יְקַבְּלוּ עֲלֵיהֶם לְהִתְעַנּוֹת כָּךְ וְכָךְ תַּעֲנִיּוֹת כְּשֶׁיַּעֲבֹר הַגְּזֵרָה.
If a city is surrounded by enemies ... the people in the city are not allowed to fast so that they won't be weakened.
It seems to me that the people shouldn't fast because they may need their strength to fight back against the surrounding enemies. So too - soldiers need their strength to fight so it would seem to me that soldiers did not and should not fast.
The fasting is the previous day of the war, not the day of the war... It is to understand that the victory is from HaShem and not for our effots.