Shechinah Dwells Through Avodah
בפסוק כ"ת מחנה כו' ונשמרת מכ"ד רע
On the pasuk "When you go out as a camp [against your enemies]... you shall guard yourself from every evil thing" (Devarim 23:10).
The Sfas Emes opens with the pasuk about Bnei Yisrael going out to war and the obligation to keep oneself far from every evil thing.
הוא ג"כ עצה ע"י הכניסה להלחם עם הסט"א זוכין להשמר מכ"ד רע
This too is a piece of counsel: through entering to wage war against the Sitra Achra, one merits to be guarded from every evil thing.
He reads this not only as a warning but as advice: the very act of going forth to battle the Sitra Achra is itself what protects a person from all evil.
ה"א מתהלך בקרב מחניך
"For Hashem your God walks about in the midst of your camp" (Devarim 23:15).
He cites the next pasuk, which states that Hashem walks about in the midst of the camp.
הל"ל מהלך
It should have said "goes" [in the simple form] rather than "walks about."
He raises a question on the wording: the pasuk uses an intensive form ("walks about") rather than the plain "goes," and asks why.
וי"ל כי הכוונה שידעו בנ"י כי השראת השכינה בעולמות תלוי בעבודת בנ"י וז"ש שתהלוכתו היא בקרב מחניך
And it may be explained that the intent is that Bnei Yisrael should know that the resting of the Shechinah within the worlds depends upon the avodah of Bnei Yisrael, and this is what is meant by saying that His going about is "in the midst of your camp."
He answers that the unusual wording teaches that the Shechinah's presence in the world is bound up with the avodah of Bnei Yisrael, so that Hashem's "walking about" is specifically within their camp.
והטעם להצילך כו' כמ"ש חז"ל בפסוק כח מעשיו הגיד כדי לתת להם נחלת גוים
And the reason [given by the pasuk] is "to rescue you..." — as Chazal expounded on the pasuk "He told the power of His deeds [to His people]" (Tehillim 111:6), that it was in order to give them "the inheritance of nations."
He connects this to the pasuk's stated purpose of rescuing them, citing Chazal that Hashem revealed the power of His deeds in order to grant Bnei Yisrael the inheritance of the nations.
שע"י שהכל נעשה על ידינו ממילא נוטלים חלק בראש:
For through the fact that everything is accomplished by means of us, it follows that we take a portion at the very head.
He concludes that since everything is brought about through Bnei Yisrael, they are the ones who receive the foremost portion.
Summary: The Sfas Emes builds on the pasuk in Ki Teitzei that commands a person going out to war to guard himself from every evil thing, reading it as counsel: the very act of going forth to do battle with the Sitra Achra is what shields a person from all evil. He then turns to the following pasuk, which states that Hashem "walks about" in the midst of the camp, and asks why the Torah uses this intensive form rather than the simple "goes." He answers that the wording teaches Bnei Yisrael that the resting of the Shechinah throughout the worlds is dependent upon their avodah, so that Hashem's movement is bound up specifically with their camp. Drawing on Chazal's exposition that Hashem revealed the power of His deeds in order to grant Bnei Yisrael the inheritance of the nations, he concludes that since everything is accomplished through Bnei Yisrael, they are the ones who take the foremost portion.