"When Sihon and all his army came out to meet us in battle at Jahaz, the Lord our God delivered him over to us and we struck him down, together with his sons and his whole army." NIV translation
Moses reminds the people of Israel that God had did what He said He would do with Sihon. They are reminded of the past faithfulness of God to prepare them for the future struggles of walking with Him in the promised land.
"At that time we took all his towns and completely destroyed them - men, women and children. We left no survivors. But the livestock and the plunder from the towns we had captured we carried off for ourselves. From Aroer on the rim of the Arnon Gorge, and from the town in the gorge, even as far as Gilead, not one town was too strong for us. The Lord our God gave us all of them." NIV translation
Moses reminded Israel that the victory over Sihon was a complete victory. They did not have to compromise or settle for a partial victory as God had given them all that He had promised. That same promise is ours in Christ. God has given us the total victory and we can have it all as long as we do not settle for less.
"But in accordance with the command of the Lord our God, you did not encroach on any of the land of the Ammonites, neither the land along the course of the Jabbok nor that around the towns in the hills." NIV translation
Moses reminds them (and us) that their success in the campaign was due to the fact that they had stayed within God's plan. They did not get proud because of their victory and then try to take what God had not given them.
This is the same for us as Christians and speaks of our contentment in Jesus Christ. We trust that God is good, He knows what is best for us, and that He wants the best for us. With this in mind, we can accept what He gives us and not envy those that have been given something different.