Sima Yi (Simplified: 司马懿; Traditional: 司馬懿) was a strategist for the Cao Wei empire during the Three Kingdoms period of China. After the collapse of Cao Wei, he started the Jin kingdom, which would later briefly reunify China as the Jin Dynasty.
Sima Yi was born into a family of scholars, famous as descendants of the historian Sima Qian. He took the exams in 204 and passed; however, he never served in the imperial court. Over the next few years, he traveled the land, seeking a new master. According to a popular account, after finishing the exams he announced "I will only lend my talents to one who deserves them. None in this court qualify."
Under Cao Cao
Around 208, Cao Cao approached Sima Yi while he was in Chang'an. Sima Yi refused at first; however, after several visits, he agreed to become minister of literature. He spent little time attending to state affairs, instead keeping track of the status of the army.
After Cao Cao died, Sima Yi approached Cao Pi to offer him his service as a strategist. Cao Pi gave him control of a force and tasked him with defeating Zhuge Liang, whom Cao Pi viewed as a threat. In 223, Sima Yi tried to capitalize on the death of Liu Bei, making small attacks to draw out Zhuge Liang. The two finally faced each other at the Battle of Wuchang Plains. While Sima Yi suffered great losses, he also forced the enemy to take the defense. Zhuge Liang died during the battle, and Sima Yi took credit for killing him.
Founding the Jin
He continued to serve under Cao Rui until the collapse of the Cao Wei empire. Before the final Shu invasion, Sima Yi gathered his family and loyal officers and escaped the capital, intent on founding a new kingdom. In many accounts, Sima Yi saboutaged the Wei forces to aid his plan, however this is likely not the case. The Jin kingdom was quietly established the next year.