Victories over massive enemies often defy all military logic. When facing powerful kings, triumph is sometimes the result of direct intervention from God rather than human strength or weaponry.
During the conquest, a mysterious force was sent ahead of the Israelite camp to clear the way. The primary approach among commentators is that this was a literal physical creature, a flying, venomous insect [מצודת דוד, מצודת ציון, רד״ק, ביאור שטיינזלץ]. This swarm would confront the enemy forces, striking their faces and blinding them. Unable to fight back, the enemies were left defenseless, allowing the Israelites to defeat them with ease [רד״ק]. Taking a slightly broader view, others suggest the phenomenon refers generally to illness, plagues, or other disasters that struck the enemy, spreading sheer terror independent of actual combat [רלב״ג, ביאור שטיינזלץ].
However, a different line of thought suggests a completely metaphorical meaning. Because the battles against the two Amorite kings, Sihon and Og [מצודת דוד, רד״ק], had already taken place earlier, this force is understood not as a physical insect or plague, but as psychological dread. In this view, the "sting" was actually the terrifying rumor of the miraculous defeat of these two mighty kings. This rumor pierced the inhabitants of Canaan, causing intense panic and driving them to flee their land [מלבי״ם].
The battles against Sihon and Og ended relatively easily, even though the immense strength of these kings should have made the conflict far more difficult [ביאור שטיינזלץ]. The natural military capabilities of the Israelites were completely insufficient to stand against such powerful rulers. Ultimately, the victory was not achieved through weapons, strategy, or combat skill, but relied entirely on the help of God [מצודת דוד, רד״ק].