Deuteronomy 2:30 & 31

"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.