He was appointed as the Grand Marshal (大司馬) when Cao Pi ascended the throne, and was also credited by the latter for the establishment of Wei.

However, Cao Ren was also once derided as a mediocre commander by Zhu Huan, a general from Wei's rival state Eastern Wu.

When chaos broke out in China towards the end of the Han dynasty, Cao Ren rallied a militia of over 1,000 men and they wandered around the region near the Huai and Si rivers.

He defeated Tao Qian's general Lü You (呂由) and led his force to rendezvous with Cao Cao's main army at Pengcheng (彭城; present-day Xuzhou, Jiangsu), where they scored a major victory over Tao Qian's forces.

Cao Ren and his cavalry later defeated Tao Qian's subordinates in various battles at the counties in Xu Province.

Cao Ren responded to the threat by defeating Han Meng at Jiluo Hill (雞洛山; located 50 li northeast of present-day Xinmi, Henan).

Besides, the city's fortifications are strong and the defenders have much supplies, so our forces would suffer heavy casualties if we attack, and we will be held up here if we switch to a defensive stance.

Cao Ren's Chief Clerk (長史), Chen Jiao (陳矯), turned pale when he saw the situation from the top of the city walls.

However, unlike what everyone thought, Niu Jin managed to create havoc around the Wu's army with just 100 men left by then.

Chen Jiao and the others attempted to dissuade him from braving danger, but Cao Ren ignored them and every soldiers in his command was ordered to charged into the enemy formation.

Cao Ren successfully rescued Niu Jin and his surviving men and even turned back to save his remaining soldiers who were still trapped in the enemy formation.

Cao Ren suppressed a rebellion led by Hou Yin (侯音) before massacring the population of Wan (宛; present-day Wancheng District, Nanyang, Henan) between late 218 and early 219,[22][23] he was then officially commissioned as General Who Attacks the South and he continued to garrison at Fan.

By then, Cao Ren and his troops had been completely isolated inside Fan as they had lost contact with outside, while their supplies were running out and there were no reinforcements in sight.

While Xu Huang was attacking Guan Yu, Cao Ren seized the opportunity to counterattack and break the siege.

Cao Ren was appointed as General of Chariots and Cavalry (車騎將軍) and was placed in charge of military affairs in Jing, Yang and Yi provinces.

[27] Cao Ren moved to Wan (宛; present-day Wancheng District, Nanyang, Henan) in northern Jing Province and garrisoned there.

Cao Ren had the general Gao Qian (高遷) oversee the relocation of civilians from the southern bank of the Han River to the north.

Later, Emperor Wen ordered Cao Ren to move to Linying (臨潁) and promoted him to Grand Marshal (大司馬).

[28] In 222, Cao Ren led several tens of thousands men to assail Sun Quan's garrison at Ruxu (濡須; north of present-day Wuwei County, Anhui), where the defending general, Zhu Huan, had only 5,000 troops remaining inside the city walls.

[29] However, Cao Ren was deemed only as an inferior commander by Zhu Huan and his colleagues, so the defenders put up a staunch defence.