“And Israel sent messengers unto Sihon king of the Amorites, saying, Let me pass through thy land: we will not turn into the fields, or into the vineyards; we will not drink of the waters of the well: but we will go along by the king's high way, until we be past thy borders.” (Numbers 21:21-22)
Six hundred years before, the Amorites had had a treaty with Abraham, as we learn from Genesis 14:13. “And there came one that had escaped, and told Abram the Hebrew; for he dwelt in the plain of Mamre the Amorite, brother of Eshcol, and brother of Aner: and these were confederate with Abram.” Israel hoped their history would result in cooperation when they wanted to cross their land toward Syria. They made the same commitment they had made to Moab, to stick to the highway and not even stop for water on the way.
“And Sihon would not suffer Israel to pass through his border: but Sihon gathered all his people together, and went out against Israel into the wilderness: and he came to Jahaz, and fought against Israel.” (Numbers 21:23)
Like the Moabites, the Amorites refused permission to cross their land. In fact, Sihon actively attacked Israel, making it clear the previous alliance no longer mattered. In Genesis 15:16, God had stated that eventually Israel would posses the land of the Amorites. “But in the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full.” During the intervening years, they had turned entirely away from God. The attack freed Israel from any lingering sense of obligation to them.
“And Israel smote him with the edge of the sword, and possessed his land from Arnon unto Jabbok, even unto the children of Ammon: for the border of the children of Ammon was strong. And Israel took all these cities: and Israel dwelt in all the cities of the Amorites, in Heshbon, and in all the villages thereof. For Heshbon was the city of Sihon the king of the Amorites, who had fought against the former king of Moab, and taken all his land out of his hand, even unto Arnon.” (Numbers 21:24-26)
Israel defeated Sihon’s army and occupied all the Amorite cities including what they had taken from the Moabites on the north side of the Arnon, establishing themselves as powerful new neighbors.
“Wherefore they that speak in proverbs say, Come into Heshbon, let the city of Sihon be built and prepared: For there is a fire gone out of Heshbon, a flame from the city of Sihon: it hath consumed Ar of Moab, and the lords of the high places of Arnon. Woe to thee, Moab! thou art undone, O people of Chemosh: he hath given his sons that escaped, and his daughters, into captivity unto Sihon king of the Amorites. We have shot at them; Heshbon is perished even unto Dibon, and we have laid them waste even unto Nophah, which reacheth unto Medeba. Thus Israel dwelt in the land of the Amorites.” (Numbers 21:27-31)
The Amorites had been far more powerful than the Moabites, so Israel’s defeat of Sihon clearly established them as a force to be reckoned with. Moab was especially fearful of their power.
“And Moses sent to spy out Jaazer, and they took the villages thereof, and drove out the Amorites that were there. And they turned and went up by the way of Bashan: and Og the king of Bashan went out against them, he, and all his people, to the battle at Edrei.” (Numbers 21:32-33)
The Amorites had also occupied the region of Jaazer but were driven out by the Israelites. From there they also turned into the land of Bashan. Og, the king, tried to stop Israel’s progress, engaging them in battle at Edrei.
Genetic giantism had been introduced as a result of cross breeding of the of the races in Genesis 6:4, but had largely disappeared by this time. Og was the last of his particular line of giants according to Deuteronomy 3:11. “For only Og king of Bashan remained of the remnant of giants; behold, his bedstead was a bedstead of iron; is it not in Rabbath of the children of Ammon? nine cubits was the length thereof, and four cubits the breadth of it, after the cubit of a man.” While he was extremely large, since his bed was over thirteen feet long and six feet wide, he was not nearly so large as depicted in most fairytales. All of the later giants appear to have been smaller, and none today are more than about eight and a half feet tall as a result of genetic dilution.
“And the LORD said unto Moses, Fear him not: for I have delivered him into thy hand, and all his people, and his land; and thou shalt do to him as thou didst unto Sihon king of the Amorites, which dwelt at Heshbon. So they smote him, and his sons, and all his people, until there was none left him alive: and they possessed his land.” (Numbers 21:34-35)
Israel had originally refused to go into Canaan because the spies had seen the three giant sons of Anak there. God specifically promised and gave victory over an even larger giant when they fought Og, the king of Bashan. It must have been a tremendous morale booster.