A.D.220 曹操died,his son,曹丕succeed to the throne, A.D.226 the son of 曹丕，曹睿 was crowned.And before he died, he entrusted the prince曹芳 to the care of 司马懿. The power of 魏 fell into 司马family‘s hand gradually. A.D.263 魏 annexed 蜀 to its territory. A.D.265 The grandson of 司马懿，司马炎(Sima Yan) forced the emperor 曹奂（another grandson of 曹操）to abdicated and became the first emperor of 晋. A.D.280 吴 fell to 晋's occupation.晋 unified the whole of China. Do you understand?

Wei never had a chance to defeat Wu. They had lots of campaigns, either side prevailed equally over the other. The decline of Wu Empire originates from the civil conflicts between two princes 孫和 and 孫霸, aka 二宮之爭 or 南魯黨爭, when the Empire Wu is still in 孫權's reign. The chaos of eight year greatly expended Wu Empire's political and military power, that it is finally defeated by Jin Empire 10+ years after the claim. Shu was acknowledged as being defeated by Wei, but the noticeable fact is it only happened two years before Jin took over Wei's throne. Besides, the defeat of Shu is due to the intrusion of Sema Zhao(司马昭).

Actually, Cao Cao unified the northern part of China during his life and became the man who's in power of the Han dynasty though he never took the throne of Han dynasty. After his death, his son Cao Pi carried on what his father(Cao Cao) had done and ended the history of Han. Cao Pi himself became the first emperor of Wei, the first and most powerful kingdom of the three kingdoms. Before the establishment of Wei, Liu Bei established Shu in the south-western part of China while Sun Quan established Wu in the east, and there were three kingdoms. We say that Wei is the most powerful one because Wei's territory contains the most rich part of China in that time and it had far more population than the other two kingdoms. Shu and Wu occupied lots of land of China too! Although Wei was so powerful, Cao Cao, Cao Pi, and Cao Pi's succesor Cao Rui(曹睿) never got the chance to unify the whole China, for Wei was not powerful enough to defeat the other two kingdoms at the same time. Shu and Wu would form an alliance while Wei became aggressive. After the death of Cao Rui, Sema Yi(司马懿) from the Sema family was in power of Wei after he defeated his rival Cao Zhen(曹真), and his family became the real rulers of Wei(though the emperor of Wei were still from Cao family). After a few years, Shu became really weak because it had spent lots of money and soldiers on campaigns against Wei(Shu in the offense), and Sema Zhao(司马昭), the real ruler of Wei sent his army to Shu and destroyed that kingdom. After that, only two kingdoms left, Wei(ruled by the Sema family, emperor from Cao family), and Wu(ruled by the Sun family and emperors from the Sun family as well). At this time Sema Yan(司马炎) took the throne of Wei and established Jin(晋). After another few years, Jin destroyed Wu and unified the whole China.(Jin is actually Wei with a new royal family, and though it was not powerful to defeat the other two kingdoms at the same time, it was powerful enough to defeat one.)

Cao Cao（曹操） tried but failed to unify the whole China. Cao Pi（曹丕）, son of Cao Cao replaced the Han dynasty by founded the Wei（魏）. After that, Liu Bei（刘备）founded the Shu（蜀）and Sun Quan（孙权）founded the Wu（吴）. There're wars and battles between the three kindoms for a long time. Wei is the most powerful while the Shu is the weakest. The Shu is conquered by the Wei. SiMa Yan（司马炎）replaced the Wei by founded the Jin（晋）. The Jin conquered the Wu and unified whole China.

Lol sorry Peter but the details do matter when you are studying Chinese history as part of your Chinese degree and have to make a presentation in 2 weeks!!