God gives Israel appetizers of the Promised Land in Deuteronomy ch.3, "for I have handed him over to you," (Deut. 3:2), the him being the King of Bashan (this came with all of his land). God then gives Israel the land of all the Amorites (Deut. 3:8), and to Joshua God says, "Your own eyes have seen everything that the Lord your God has done to these two kings; so the Lord will do to all the kingdoms into which you are about to cross. Do not fear them, for it is the Lord your God who fights for you" (Deut. 3:21-22).
Amongst all of this, the beginning of promise, Moses makes a plea to God, "O Lord God, you have only begun to show your servant your greatness and your might; what god in heaven or on earth can perform deeds and mighty acts like yours! Let me cross over to see the good land beyond the Jordan, that good hill country and Lebanon," (Deut. 3:23-24). YHWH says no (Deut. 3:26). To understand why God might reply so harshly in a time of so much blessing we have to return to Deuteronomy ch. 1 where God first tells Moses that he is not to enter the Promised Land (v.37-38). Here we again witness the reality of sin. Sin destroys community, and creation but the worst effect is the offense it is to God. Think of it like this: when you offend someone do you always understand what, particularly, offended them? My response is often: "Are you serious, that little thing offended them?" What this tells us is that we are, at some level, detached from other people; we don't understand each other's hearts well enough to live in perfect community. It is this way with our relationship with God too. Sin is the ultimate offense to God. Oftentimes, we read the Old Testament or the Passion narrative in the Gospels and are left asking, "is sin really that big of a deal?" I believe that the answer is a hearty yes, the image we have signifying this reality is the cross of Christ; other than the crucifixion we have no frame of reference for establishing the level of offense sin is to God.
So when Moses asks God if he has changed his mind about allowing him entrance into the Promised Land God replies with a harsh "No". Here God isn't being a bully, no, rather Moses is failing to understand the gravity of sin, and his failure at keeping Israel out of sin (see chapter one). With that in view, we can see that God's grace lasts much longer than his anger (Deut 5:8-10) and his promise of the Promised Land remains a promise for Israel through the Hebrew Bible and on into the intertestamental period until Jesus initiates a new promised land in himself.