Hashem Directs Every War
בפסוק כי תצא למלחמה כו' סוס ורכב כו' ה"א עמך המעלך כו' ההולך עמכם כו' להושיע אתכם
On the verse, "When you go out to war against your enemies, and you see horse and chariot, a people more numerous than you, do not fear them, for Hashem your God is with you, who brought you up from the land of Mitzrayim" (Devarim 20:1), and similarly "who goes with you... to save you" (Devarim 20:4).
The Sfas Emes opens with the parsha's verses about going out to war, noting that Hashem is described as the One who goes with Bnei Yisrael to save them.
כבר כ' במ"א כי לשון ההולך משמע שכל הסיבה של המלחמה שסיבב הש"י הכל לטובתנו
We have already written elsewhere that the expression "who goes" implies that the entire cause of the war, which Hashem brought about, is all for our benefit.
He cites his own earlier teaching that the word "goes" reveals that Hashem Himself orchestrated the entire war, and did so entirely for the good of Bnei Yisrael.
וז"ש להושיע אתכם שכל המלחמה הי' בעבור הישועה
And this is the meaning of "to save you" — that the whole war was for the sake of the salvation.
The phrase "to save you" teaches that the war itself existed only as a vehicle for bringing about the salvation.
וכמו שהי' ביצ"מ הכנה לקבל מלכותו ית' כמ"ש אשר הוצאתי כו' להיות לכם לאלקים כן הוא לעולם
And just as at Yetzias Mitzrayim it was a preparation to receive His blessed kingship, as it is written, "I am Hashem who brought you out... to be a God to you" (Bamidbar 15:41), so it is for all time.
Just as the redemption from Mitzrayim was a preparation for Bnei Yisrael to accept Hashem's kingship, the same purpose underlies every situation for all generations.
וז"ש ה' ילחם לכם לטובתכם ולהנאתכם יגרום מלחמות
And this is the meaning of "Hashem will fight for you" (Shemos 14:14) — for your benefit and for your good He brings about wars.
When the Torah says Hashem fights for Bnei Yisrael, it means He arranges wars themselves for their ultimate benefit and good.
וז"ש ה' איש מלחמה
And this is the meaning of "Hashem is a Man of war" (Shemos 15:3).
This is also the meaning of calling Hashem "a Man of war."
לבד שהוא מושיע במלחמה אבל כל תהלוכת המלחמה מאתו ית' וכשהוא בדרך זה נק' איש מלחמה שהוא מתלבש בהטבע וזה ענין ימי המעשה
Beyond the fact that He saves in war, rather the entire conduct of the war is from Him, may He be blessed; and when He acts in this manner He is called "a Man of war," for He clothes Himself within nature, and this is the matter of the weekday days.
Hashem does not merely rescue within a war that runs on its own — the entire course of the war flows from Him; when He operates this way, hidden within nature, He is called "a Man of war," which corresponds to the weekday.
ה' שמו הוא בהתגלות ביום השבת שנקרא יום מנוחה וז"ש שמע ישראל כו' פירש"י אפי' אין בכם רק זכות ק"ש כו'
"Hashem is His Name" (Shemos 15:3) — this is in revelation on the day of Shabbos, which is called a day of rest; and this is the meaning of "Shema Yisrael" (Devarim 6:4), as Rashi explains, "even if you have within you only the merit of Krias Shema," and so on.
By contrast, "Hashem is His Name" reflects His open revelation on Shabbos, the day of rest, which connects to Krias Shema — where Rashi says even the bare merit of Shema can stand for a person.
פי' זה שמאמינים באחד האמת שהכל הוא מאתו ית' והוא מנהיג כל הטבע עי"ז יש לנו לבטוח בו ית' שהכל לטובתנו כנ"ל:
The meaning is that through believing in the one true Hashem — that everything is from Him, may He be blessed, and that He directs all of nature — through this we have reason to trust in Him, may He be blessed, that everything is for our benefit, as above.
The point is that recognizing the one true Hashem, who is the source of everything and directs all of nature, gives us the grounds to trust that everything He brings is for our benefit.
Summary: The Sfas Emes explains the parsha's account of going out to war by stressing that Hashem is the One who "goes with" Bnei Yisrael, meaning He Himself orchestrates the entire war, and entirely for their good. Just as Yetzias Mitzrayim was a preparation to accept Hashem's kingship, so too every war and circumstance is ultimately a vehicle for salvation and for receiving His malchus. The name "Hashem is a Man of war" reflects how He clothes Himself within nature during the weekday, directing the whole course of events from behind the scenes, while "Hashem is His Name" reflects His open revelation on Shabbos, the day of rest. This corresponds to Krias Shema, where even its bare merit can stand for a person. The avodah is to believe in the one true Hashem, the source of everything and the Director of all nature, and through that emunah to trust that whatever He brings about is entirely for our benefit.