The confrontation with Og, King of Bashan, presented the leader of the Israelites with an unprecedented threat. The sudden appearance of this ancient enemy sparked a deep anxiety that required direct divine intervention to calm hearts before the battle. Unlike the previous conflict with Sihon, Moses needed explicit reassurance from God not to be afraid. The primary approach among commentators is that this fear stemmed from a combination of tactical, physical, and, most importantly, spiritual factors.
On a practical level, Og launched a sudden, unprovoked attack on his own initiative, without God having issued a prior command to fight him [העמק דבר, מלבי״ם]. Physically, Og was an ancient giant of immense size and strength [ברכת אשר על התורה]. According to tradition, his proportions were monstrous, and he even attempted to annihilate the Israelites by hurling an entire mountain upon them [צאינה וראינה].
Yet, Moses's primary concern was spiritual. His dread was not merely a reaction to physical might, but a profound awe of the enemy's historical merits [ברכת אשר על התורה]. Moses worried about two specific acts from Og's past. First, Og was the survivor who originally informed Abraham that his nephew Lot had been taken captive [רש״י]. Although Og harbored malicious intentions, hoping Abraham would be killed in battle so he could marry Sarah, the fact that his actions led to Lot's rescue granted him spiritual credit. Second, Og had served Abraham and was circumcised by him. Moses feared this specific merit would protect the giant, especially since the generation of Israelites born in the desert remained uncircumcised at that time [אלשיך]. Moses could not simply rely on his own righteousness or that of the Israelites, knowing that God rewards the good deeds of the wicked in this current world, while reserving the reward of the righteous for the world to come [ברכת אשר על התורה].
In response to these heavy concerns, God reassured Moses by declaring that Og's defeat was already an accomplished fact, sealed generations earlier during the time of Abraham. When Og attended the feast celebrating Isaac's circumcision, he mocked the infant and boasted that he could crush the child with a single finger. At that moment, it was decreed that he would ultimately fall to Isaac's descendants [צאינה וראינה, אלשיך].
God specifically emphasized that Og would be delivered directly into Moses's hand. This was a personal message: Moses, who was circumcised himself and had circumcised the Israelites upon leaving Egypt, possessed the spiritual merit of the covenant necessary to overpower Og's merit [אלשיך]. Furthermore, God promised that Og would suffer the exact same fate as Sihon. On a military level, this meant that just as Sihon had abandoned the safety of his fortified cities, making them vulnerable to conquest, Og would make the same fatal tactical error [מלבי״ם]. On a spiritual level, the comparison taught a deeper principle. Just as Sihon's city of Heshbon lost the spiritual protection of Lot's merit the moment it was conquered from Moab, Og forfeited all his historical merits the moment he abandoned Abraham's path and defiled his covenant [אלשיך].