God had made his spirit stubborn and his heart obstinate (literally God hardened his spirit and strengthened his heart): Like Pharaoh, Sihon refused to repent and encountered God's wrath (see Exodus 7:13). God knew that showing more grace to these rulers would be of no use.
There is mystery in how people harden their own hearts (for example, Exodus 7:13, 22; 8:15) and how God hardens hearts (for example, Exodus 4:21; 7:3; 9:12). What is clear is that God asks for repentance and is ready to forgive. Yet, when people keep ignoring or rejecting him, they may become unable to hear and obey God (see “Hardened Hearts” Theme Note; Romans 1:21–28; 9:17–24).