The First Intermediate Period is the name conventionally given by Egyptologists to that period in Ancient Egyptian history between the end of the Old Kingdom and the advent of the Middle Kingdom. As such, depending on when individual historians place the 'downfall' of the Old Kingdom - with the end of either the Sixth or the Eighth Dynasties - the First Intermediate period (sometimes abbreviated as 'FIP') can be considered to embrace the Seventh, Eighth, Ninth, Tenth and most of the Eleventh Dynasties.
The Old Kingdom was weakened by famine and weak leadership. One theory holds that a sudden, unanticipated, catastrophic reduction in the Nile floods over two or three decades, caused by a global climatic cooling which reduced the amount of rainfall in Egypt, Ethiopia and East Africa, contributed to the great famine and the subsequent downfall of the Old Kingdom.
The last Pharaoh of the 6th dynasty was Pepi II who probably reigned for 94 years, longer than any monarch in history. He was 6 when he ascended the throne and 100 years old when he died. The latter years of his reign were marked by inefficiency because of Pepy's advanced age. When he died the Old Kingdom collapsed.
A dark time marked by unrest followed. The Union of the Two Kingdoms fell apart and regional leaders had to cope with the famine.
Around 2160 BC a new line of Pharaohs tried to reunite Lower Egypt from their capital in Heracleopolis. In the meantime, however, a rival line based at Thebes was reuniting Upper Egypt and a clash between the two rival dynasties was inevitable.
The Pharaohs from Herakleopolis descended from a Pharaoh named Akhtoy and the first four Pharaohs from Thebes were named Inyotef or Antef.
Around 2055 BC a descendant of Inyotef defeated the Heracleopolitan Pharaohs, reunited the Two Lands, founded the 11th dynasty and ruled as Mentuhotep II thereby ending the First Intermediate Period.