As punishment for striking the rock instead of speaking to it (Numbers 20), God told Moses that he would not be allowed to enter Canaan.
That’s a hard blow, but Moses’ sin is understood better in the immediate context. He just lost Miriam (Numbers 20:1), he was about to lose Aaron (Numbers 20:23-29), and the people were relentless in their accusations that Moses and his team led Israel out in the wilderness to die (Numbers 20:2-7). Clearly, he was getting past the end of his rope.
Still, a sin is a sin, and nobody knew that better than Moses.
That doesn’t mean he can’t at least ask God to change His mind though, right? After all, God “relented” in His destruction of the people after the Golden Calf incident (Exodus 32), after the spies’ bad report (Numbers 14:20).
God also stopped an ongoing destruction at the request of Moses in the cases of the people, Miriam, and Korah (Numbers 11, 12, 16). Couldn’t He do that for Moses, too?
Reading through this section again, it seems that’s exactly what Moses hoped for. The language in Deuteronomy 3:24 is interesting: “Lord, You have begun to show Your servant Your greatness and Your strong hand…”
Begun. Even though God’s strength had been on display for more than forty years by this point, in Moses’ view, this strength was best utilized in fulfilling the promise of Canaan. And He had only begun that process; the destruction of Sihon and Og were a glimpse into His dealings with the Canaanites (Deuteronomy 2:31; 3:21).
It’s even possible that Moses viewed these first two victories over Sihon and Og as a tacit admission from God that all was forgiven—that if Moses was allowed to see the beginnings of the conquest, maybe he could see the rest?
Unfortunately for him, God slams the door on that request. The phrase God uses in Deuteronomy 3:26 (“Enough!”) is almost the exact same word that He says to the people in Deuteronomy 1:6 (“You have stayed long enough at this mountain!”). In both cases, God wants to silence the matter.
That’s not to say that Moses is completely unheard. God does allow him to “go up” to Pisgah, which, in a way, is all he really wanted. Moses might have hoped to set foot across the Jordan, but at least he gets to see the land where his ancestors will live.
Deuteronomy is an emotional sermon from Moses. His angst in seeing the land is channeled through his accusations that Israel was partially to blame for his own failings. While he should bear the brunt of his own sin, their constant murmurings drove Moses past the breaking point.
If they wanted solid leadership in the future, it begins with not constantly harassing them at every turn. In the end, it might end up costing everyone more than they know.