Inner Battle Rectifies Sin
כי תצא למלחמה כו' עם רב ממך כו'
"When you go out to war... a people more numerous than you..." (Devarim 20:1).
The piece opens with the verse describing Bnei Yisrael going out to war against an enemy that seems larger and stronger than they are.
באוה"ח פי' שכוחות סט"א הם ממך כו'
In the Ohr HaChaim it is explained that the forces of the sitra achra (the Other Side) are themselves drawn from you.
The Ohr HaChaim teaches that the strength the yetzer hara wields against a person is actually borrowed from that person's own spiritual energies.
ויאמר הכתוב כי לא תירא מזה כי ה"א עמך המעלך מא"מ
And the verse states that you should not fear this, "for Hashem your God is with you, who brought you up out of the land of Mitzrayim" (Devarim 20:1).
The Torah reassures us not to be afraid, because Hashem, who redeemed us from Mitzrayim, stands with us in this struggle.
פי' כי זה סימן לטובה שאין הקב"ה מניח האדם בחטאיו רק ע"י החטאים יש לו מלחמות בכל דבר מצוה ותפלה
The meaning is that this itself is a sign for good, for the Holy One, Blessed is He, does not abandon a person in his sins; rather, it is precisely through the sins that he is given battles in every matter of mitzvah and tefillah.
The Sfas Emes reframes the battle as good news: the very existence of the struggle in mitzvos and tefillah shows that Hashem has not given up on the person but is engaging him so his sins can be addressed.
ולכן עי"ז שמתגבר בעבודת הבורא נגד המ"ז
Therefore, through the very fact that he strengthens himself in the avodah of the Creator against the yetzer hara,
When a person musters strength in serving Hashem and pushes back against the yetzer hara,
עי"ז זוכה לתקן החטאים
through this he merits to repair the sins.
that exertion is itself the instrument through which his sins are repaired.
וכן הי' במצרים שכל הגלות הי' לטובה כי בנ"י הוצרכו לתקן החטאים
And so it was in Mitzrayim, that the entire galus was for the good, for Bnei Yisrael needed to repair the sins.
He brings a proof from Mitzrayim, where the whole purpose of the galus was ultimately for the good, since Bnei Yisrael needed to fix their sins.
וע"י הגלות נתקנו כל החטאים שהיו בכלל נפשותיהם כידוע מסה"ק:
And through the galus all the sins that were included within their souls were rectified, as is known from the holy seforim.
The hardship of the galus served to rectify all the sins bound up within their souls, a concept known from the holy seforim.
Summary: Building on the verse about going out to war against a seemingly stronger foe, the Sfas Emes, citing the Ohr HaChaim, explains that the power of the sitra achra is itself drawn from a person's own strengths. Far from a reason to fear, this struggle is a sign of Hashem's good will, for He does not abandon a person to his sins but instead gives him battles within every mitzvah and tefillah. By rising up in the avodah of Hashem against the yetzer hara, a person merits to repair those very sins. So too the galus in Mitzrayim was entirely for the good, since Bnei Yisrael needed to rectify their sins, and through that galus all the sins embedded in their souls were repaired, as is known from the holy seforim.