An eternal salvation against eternal evil

בנוסח דשושנת יעקב תשועתם היית לנצח ותקותם בכל דור.

In the text of "Shoshanas Yaakov": "You were their salvation forever, and their hope in every generation."

The Sefas Emes takes up the wording of the Purim hymn, which speaks of Hashem's salvation as something that endures "forever" and a hope present in "every generation."

כי המן ועמלק היו רשעים להרע לכל הדורות כמו שראינו שגזר לאבד כל היהודים מכל הדורות.

For Haman and Amalek were wicked, bent on harming all the generations, as we saw that he decreed to destroy all the Jews of all generations.

The evil of Haman and Amalek was not limited to one moment; Haman's decree aimed to wipe out the Jewish people of every generation entirely.

ובכל דור חייב אדם לראות כאלו גזר על זה הדור.

And in every generation a person is obligated to see it as though the decree was issued against this very generation.

Because the threat spans all generations, each person in every era must regard Haman's decree as if it were aimed directly at his own time.

כמו שאומרים ארור המן אשר ביקש לאבדי דייקא.

As we say, "Cursed is Haman who sought to destroy me" — precisely "me."

The liturgy's wording "to destroy me" is exact: every individual is meant to feel personally targeted by Haman's plot.

ולעומת זה היה מרדכי הצדיק יסוד עולם לכל הדורות.

And corresponding to this, Mordechai the tzaddik was a foundation of the world for all the generations.

As a counterforce to Haman's all-generational evil, Mordechai stood as a tzaddik whose merit sustains the world across all generations.

כמ"ש דובר שלום לכל זרעו.

As it is written, "speaking peace to all his offspring."

The pasuk (Esther 10:3) describes Mordechai as one whose good extended to all his descendants — that is, to all future generations.

שיש צדיק לשעה.

For there is a tzaddik for the hour.

Some tzaddikim have an influence that belongs to their own moment in time.

ויש צדיק קבוע לדורות.

And there is a tzaddik fixed for all generations.

But Mordechai was the second kind — a tzaddik whose spiritual force is permanently established for every generation.

ולפי שהמן הרשע רצה לאבד כל הדורות.

And because the wicked Haman wished to destroy all the generations.

Precisely because Haman's evil reached toward all generations.

נתן לנו הקב"ה ישועה בימים אלו לכל הדורות כמ"ש והימים כו' נזכרים כו' בכל דור כו':

The Holy One, blessed be He, gave us a salvation on these days for all the generations, as it is written, "and these days... are remembered... in every generation," etc.

To answer evil that spanned every generation, Hashem made the salvation of Purim equally eternal — the days are "remembered and observed in every generation," a redemption that holds for all time.

Summary: Because Haman and Amalek's evil reached toward every generation, the Purim salvation was made correspondingly eternal — with Mordechai standing as a tzaddik fixed for all generations and the days of Purim remembered and effective in every generation forever.