Later in 620, Emperor Gaozu sent his son Li Shimin the Prince of Qin to attack Wang Shichong the Emperor of Zheng, whose capital was at Luoyang. Feng accompanied Li Shimin as one of his strategists. In 621, Dou came to Wang's aid, and Emperor Gaozu, initially believing that Li Shimin could not prevail under those circumstances, secretly ordered retreat. Instead of retreating, Li Shimin sent Feng back to Chang'an to analyze to Emperor Gaozu that Wang was in desperate straits, but that if allowed to survive, could regroup and become a serious threat later. Emperor Gaozu was persuaded and allowed Li Shimin to remain. Li Shimin subsequently defeated and captured Dou, and Wang surrendered, allowing Tang to defeat two major enemies simultaneously. Pleased, Emperor Gaozu praised Feng, comparing his advice to that of the Jin Dynasty official Zhang Hua, whose advice had permitted Emperor Wu of Jin to destroy Eastern Wu. For his contributions, Feng was created the Duke of Pingyuan and continued to serve as Li Shimin's assistant.