Creating an army is not the traditional Jewish response.
The Mishnah Brurah (686:2 quoting the Rambam) writes:
[We] fast on the thirteenth of Adar [Taanis Ester]. Because in the days of Mordechai and Ester, [Jews] were gathered to make war and
stand on their lives on the thirteenth of Adar, and they needed to request mercy and [they
would] supplicate that G-d would help them to get revenge from their
enemies. And we find that when it was the day of the war, they would
fast, because our rabbis z"l said that Moshe Rabeinu a"h — on the day
that he fought with Amalek — was fasting. And if so, certainly in the
days of Mordechai they fasted on that day! And therefore, all of
Israel had the minhag to fast on the thirteenth of Adar, and it is
called Taanis Ester, in order to remember that Hashem y' watches and
listens to every man at the time of his need, when he fasts and
returns to Hashem with all of his heart, like they did in those days.
We can take a few messages from this:
The Jews were gathered on the thirteenth of Adar. They had known that there would be a war on the thirteenth of Adar since Pesach of the previous year (11 months), and only now they gathered their army!
They fasted. Fasting makes a person weak, but it helps a person do teshuvah. Doing teshuvah is more important than military strength.
I will conclude with a quote from Tehilim (20:8-9): "These with chariots, and these with horses, but we will mention Hashem's name [pray (Rashi)]. They bent and fell down, but we came up and overpowered."
Metzudas David: We had few horses and chariots.
Malbim: We had no horses and chariots.