"From the Desert of Kedemoth I sent messengers to Sihon king of Heshbon offering peace and saying, 'Let us pass through your country. We will stay on the main road; we will not turn aside to the right or to the left. Sell us food to eat and water to drink for their price in silver. Only let us pass through on foot - as the descendants of Esau, who live in Seir, and the Moabites, who live in Ar, did for us - until we cross the Jordan into the land the Lord our God is giving us.'" NIV translation
Moses continues his parting words to the Israelites by telling of his shortcomings in following God. As we see here, he did not do as God said and simply lead the Israelites against Sihon and his army. Instead, he did what seemed safe and right to his thinking by sending messengers to the king. Moses' actions may seem like a good thing but they were not what God had told Israel to do. We often forget that Moses was a simple man much like we are. He had his own weaknesses and failings in faith but God still used him in a mighty way.
In the same way, God can use our failures as a testimony to others.
"But Sihon king of Heshbon refused to let us pass through. For the Lord your God had made his spirit stubborn and his heart obstinate in order to give him into your hands, as he has now done. The Lord said to me, 'See, I have begun to deliver Sihon and his country over to you. Now begin to conquer and possess his land.'" NIV translation
God knew what was in the heart of Sihon even though Moses did not. Therefore, when the offering of peace was presented to him, he refused and the true nature of his heart was revealed.
This is a reminder to us, as Christians, that we are to seek God's guidance in our dealings with the people of this world. As Jesus said in the Sermon on the Mount (see Matthew 7), we are to seek God's guidance before we end up casting our pearls to pigs. In a sense, Moses was casting the pearl of peace to the pig and it was trampled underfoot.