In the Keriah which we read on Purim, the Torah states,ויאמר ה' אל משה כתב זאת זכרון בספר... כי מחה אמחה את זכר עמלק מתחת השמים, “Hashem said to Moshe, ‘Write this as a remembrance in the book… that I shall surely erase the memory of Amalek from under the heavens’ (Shemot 17:14).”
What horrible thing did Amalek, do that a nation like Midyan did not? If anything, Midyan deserves a harsher punishment, as they were the ones who tried to get Bnai Yisrael to do Avoda Zara. The real difference, however, is the manner in which they went about attacking Bnai Yisrael. Midyan, recognizing the power of Hashem, first asked Bilam to curse Bnai Yisrael so that they would have a chance at winning against them. However, after failing, Bilam suggested that they should try to cause Bnai Yisrael to sin. In both situations, Midyan understood that Bnai Yisrael was strong since Hashem was with them. Amalek, on the other hand, attacked us with full disbelief in Hashem, right after the Ten Plagues and קריאת ים סוף. Therefore, such a nation, as the Pasuk says, should be expunged from the world.
We see this at the end of the reading for Purim where it says מלחמה לה' בעמלק מדר דר, “Hashem fights a war against Amalek, from generation to generation” (Shemot 17:16). One of the three commandments regarding Amalek, is to annihilate their remembrance from the Earth תמכה את זכר עמלק. In order to accomplish this, we need to look for Amalek in our day. This generation’s Amalek is probably the easiest to recognize. Terrorists, no matter who they are, attack us with full disbelief in the true Hashem. They do not try to cause Bnai Yisrael to sin, but instead they attack for no real purpose at all. Maybe, if we help in this war on Amalek that we are engaged in right now, we will be able to finish that promise of Hashem; to expunge Amalek מתחת השמים.