I heard this last year from R’ Shalom Axelrod. Hopefully, I will do it justice.
There are two times in the Torah that Parshas Zachor, the story of Amalek, is discussed at length. However, they are each preceded by two distinct parshas.
In Parshas Beshalach (which we read this morning), Parshas Zachor comes right after the Jews complained about the lack of water. They questioned where Hashem was. Immediately after this, Amalek attacks.
In Parshas Ki Seitzei (which we read yesterday), Parshas Zachor comes right after the laws of being honest in business (the Torah teaches us not to have two different sized weights so we don’t come to cheat anyone).
The Kli Yakar explains that these are precisely the two times that Amalek comes to destroy us; when we have a lack of emuna (as the Jews did when they complained about the lack of water) and when we are not honest and united with our fellow Jews.
In the Megilla, it is stated that Haman approaches Achashveirosh and explains why they should take this opportunity to annihilate the Jews once and for all. One of the reasons he gives Achashveirosh is that the Jews are “Miphurod bein haamim” which means dispersed or “divided among the people.” As a descendant of Amalek, Haman knew that this was an opportune time to strike the Jews since they were divided, and when the Jews are divided, that is a time that Amalek can destroy them.
Interestingly, if you look at the pasuk right before the parsha of Amalek in Beshalach, it tells us that this all took place in Rephidim. Rephidim has the same root as “miphurod”, the word Haman used in the Megilla to describe the Jews as “divided.” Therefore, what was Esther’s solution to Haman’s evil decree to destroy the Jews? She said to gather the Jews together, fast together and daven to Hashem. That is because when we have Emuna and our Bein Adam Lechaveiro (matters between each other) is where it should be, Amalek cannot harm us.