במדבר, פרק כ״א, פסוק כ״ד

The triumph of the Israelites over Sihon, king of the Amorites, resulted in a massive territorial expansion, but it also brought complex geographical and legal boundaries to the forefront. The defeat of the Amorite king was absolute [ביאור שטיינזלץ], yet it was executed strictly in accordance with Torah law [צפנת פענח]. The nature of this takeover was unique; tradition notes that the Israelites took possession of the land as a singular entity, hinting that this was not a standard, permanent historical conquest [צפנת פענח].

The newly acquired land stretched from the southern edge of the kingdom to its northern limit [העמק דבר]. The southern boundary was marked by the Arnon river, which served as the border of Moab [ביאור שטיינזלץ]. The Israelites claimed the territory starting from the far side of the river, deliberately leaving the river itself outside their domain [ביאור יש״ר, הכתב והקבלה]. To the north, the Jabbok river functioned as a clear, natural boundary [דעת זקנים, בכור שור, הדר זקנים].

Expanding the territory up to the border of the Ammonites presented a unique challenge, as God had explicitly forbidden the Israelites from waging war against Ammon and Moab. The primary approach among commentators is that this divine prohibition applied only to lands actively controlled by Ammon and Moab at that time. Because Sihon had previously captured this specific territory from them, the Israelites were permitted to conquer it directly from him. This principle meant that the prior Amorite conquest had essentially prepared the ground, legally clearing the way for the Israelites to claim it [רבנו בחיי, אבן עזרא, חזקוני, ביאור יש״ר, צפנת פענח, רש״ר הירש].

At the edge of this expansion, the Ammonite border is described as being exceptionally strong, prompting different views on the nature of this strength and who it held back. The primary approach among commentators is that the border was physically and geographically formidable against Sihon. Although Sihon was a powerful conqueror, he could not overtake the rest of the Ammonite land because the Jabbok river possessed a fierce, rushing current. This created an impassable natural barrier that forced his armies to halt [שד״ל, דעת זקנים, בכור שור, חזקוני, צפנת פענח, ביאור יש״ר, רש״ר הירש, פענח רזא, הדר זקנים].

Conversely, another approach suggests that the strength of the border was not physical, but rather spiritual and legal, acting as a barrier specifically against the Israelites. In this view, the true power of the boundary was God's stern warning against attacking Ammon, which successfully stopped the Israelites from advancing any further [רש״י, רבנו בחיי]. This perspective is supported by the logic that if the Ammonites had truly been physically stronger than Sihon, the surrounding nations would have paid tribute to them instead of the Amorite king [משכיל לדוד].