The Middle Kingdom was the period of Ancient Egypt that spanned from c. 1980 to 1630 BC and encompassed the Eleventh to the Fourteenth Dynasties. It was known as the period of reunification of Egypt.
After conquering the Tenth Dynasty, who ruled Lower Egypt, Mentuhotep II was able to reunify the whole of Egypt, which started the Middle Kingdom. His son and grandson, Mentuhotep III and Mentuhotep IV, continued this process by solidifying their predeccessor’s gains.
Amenemhet I was the first king of the Twelfth Dynasty and seems to have usurped Mentuhotep IV, with a possible civil war fought between them. The kings of the Twelfth Dynasty implemented a policy of fortifying the borders of Egypt in the Delta, the Sinai Peninsula and the First Cataract. They also expanded the territory and influence of Egypt with campaigns in Nubia to the south under Senusret I and Senusret III.
Domestically, administrative reform took place under Amenemhet I and Senusret III, who would now appoint the high offices of state themselves, with a need to rein in the nomarchs, governors of the Egyptian nomes, who had gained considerable power during the First Intermediate Period, and were now hereditary positions. This was also a period of economic growth with the irrigation of the Fayum under Senusret II and semi-permanent mining in the Sinai and Wadi Hammamat under Amenemhet III. The Twelfth Dynasty came to an end after the reign of Sobekneferu, the first historically attested female king of Egypt, as did the Golden Age.
The Thirteenth Dynasty marked the gradual decline of the Middle Kingdom. There is scarce evidence for most of the kings of this dynasty but it seems that they were able to control a unified Egypt up until the end of the reign of Neferhotep I. After this, the Fourteenth Dynasty took over parts of the Nile Delta and ruled in their own right at the same time as the Thirteenth Dynasty.
While the Middle Kingdom was viewed as one of the Golden Ages of Ancient Egypt, it was followed by a period of disunity and relative cultural decline known as the Second Intermediate Period.