Conquering Canaan Through Moshe
בפסוק וישמע הכנעני דרשו חז"ל שמע שמת אהרן ונסתלקו ענני הכבוד וקסבר ניתן רשות להלחם בישראל
On the verse "And the Canaanite heard" (Bamidbar 21:1), Chazal expounded that he heard that Aharon had died and that the Ananei HaKavod (Clouds of Glory) had departed, and he held the view that permission had now been granted to wage war against Bnei Yisrael.
Chazal read the Canaanite's attack as triggered by the passing of Aharon, which removed the Ananei HaKavod and seemingly opened the way to wage war against Bnei Yisrael.
נראה דבאמת הי' המכוון להיות להם קצת כח לבוא להלחם
It appears that in truth the intent of Hashem was that they should have a measure of strength to come and wage war against Bnei Yisrael.
The Sfas Emes reframes this: the opening for the enemy was not a misfortune but part of the plan of Hashem, who deliberately granted the Canaanites just enough strength to come out and fight.
כי בהיותם תחת הענן לא היו יכולין להלחם כלל
For so long as Bnei Yisrael were beneath the Cloud, the nations were entirely unable to make war upon them.
While the Cloud surrounded Bnei Yisrael, no nation could even approach to fight them, so this protection had to lift for the coming conquests to begin.
ורצונו ית' הי' שימסרו תחת יד משה רבינו ע"ה
And the will of Hashem Yisbarach was that these nations should be delivered into the hand of Moshe Rabbeinu, alav hashalom.
The will of Hashem in this was that these enemy nations should fall specifically into the hand of Moshe Rabbeinu and his generation.
וזה הי' שורש והתחלה לכבוש כל מלכי כנען
And this was the root and the beginning of the conquest of all the kings of Canaan.
This initial encounter served as the seed and starting point from which the entire conquest of Canaan would unfold.
ולכן אמרו בנ"י אם נתן תתן כו' שהבינו כי בזו הנתינה יכבשו אח"כ כולם
Therefore Bnei Yisrael declared, "If You will indeed deliver this people into my hand" (Bamidbar 21:2), for they understood that through this one act of delivering-over they would afterward conquer them all.
The vow of Bnei Yisrael upon being attacked reflects their grasp that defeating this one foe would set in motion the conquest of all the Canaanite kings.
ורמזו בגמ' הוא סיחון ערד כנען
And Chazal alluded to this in the Gemara when they said: he is Sichon, he is Arad, he is the Canaanite - that these were one and the same.
The Gemara identifies Sichon, Arad, and the Canaanite as one and the same, hinting that these names point to a single underlying enemy bound together as one chain.
שדומה לסייח במדבר
For he is compared to a sayach (a young foal) in the wilderness.
The comparison to a young foal darting in the wilderness conveys that this enemy was wandering and rootless, appearing suddenly out of its proper place.
ערוד במדבר מה שמו כנען
As in the riddle, "The wild ass (arod) in the wilderness, what is its name? Canaan" - the names are bound together as one.
Drawing on the Gemara's riddle equating the wild ass of the wilderness with Canaan, the Sfas Emes shows these identities are deliberately fused into one.
פי' שעיקר ישיבתם הי' בכנען אבל קפצו כאן בדרך מקרה כסייח וערוד במדבר בלי ישוב וקביעות, והי' כדי שיהי' להם מפלה בכח הדור של משה רבינו ע"ה כנ"ל
The meaning is that the essential dwelling-place of these nations was in the land of Canaan, but here they merely sprang up incidentally by way of a chance occurrence, like a foal or a wild ass in the wilderness, without settled habitation or permanence; and this came about so that they should suffer their downfall through the strength of the generation of Moshe Rabbeinu, alav hashalom, as explained above.
Their true home was Canaan, but their appearance here in the wilderness was transient and unsettled - a situation arranged by Hashem so that they could be brought down by the spiritual strength of the generation of Moshe Rabbeinu.
וכן כתיב לסיחון כו' ולעוג כו' ולכל ממלכות כנען משמע שבמפלת סיחון ועוג נמסרו אח"כ כולם ביד בנ"י:
And likewise it is written, "to Sichon... and to Og... and to all the kingdoms of Canaan" (cf. Tehillim 135:11) - which implies that with the downfall of Sichon and Og, all of them were afterward delivered into the hand of Bnei Yisrael.
The verse joining Sichon, Og, and all the kingdoms of Canaan shows that the downfall of these first foes secured the eventual delivery of all the rest into the hand of Bnei Yisrael.
Summary: The Sfas Emes reinterprets the Canaanite's attack that followed the passing of Aharon and the departure of the Ananei HaKavod. Rather than seeing the loss of this protection as a setback, he explains it as the deliberate will of Hashem: only once the Cloud lifted could the enemy nations engage Bnei Yisrael and thereby be delivered into the hand of Moshe Rabbeinu, making this first battle the root and beginning of the entire conquest of Canaan. The vow of Bnei Yisrael upon being attacked reflects their understanding that this single victory would cascade into the defeat of all the Canaanite kings. Drawing on the Gemara that identifies Sichon, Arad, and the Canaanite as one - and on the riddle linking the wild ass of the wilderness to Canaan - he shows these enemies were rootless wanderers, displaced from their true home so that they could be overthrown by the spiritual strength of the generation of Moshe Rabbeinu. The closing verse, joining Sichon, Og, and all the kingdoms of Canaan, confirms that the downfall of the first foes secured the eventual conquest of them all by Bnei Yisrael.