The Half-Millennium of Persian History Between The Coming of the Saljuqs and the establishment of the Safavid dynasty is one of repeated upheaval and largely alien rule. The arrival of the Ghuzz tribes in the early 11th century was not an entirely peaceful affair—the author of the Tārīkh-i Sīstān regards it as a calamity for that formerly prosperous province—and much worse was to follow, with the Mongol invasions of the 13th century and Timur's campaigns in the late 14th, all of which caused enormous destruction, while leaving a profound impression on Persian society, culture, and political life.
This long and eventful period is seldom treated as a whole; volume 2 of Marshall Hodgson's The Venture of Islam is still perhaps the nearest approach to a united vision of the “Middle Periods” of Islamic history, but of course his view is not confined to Iran. Traditionally, the Mongol sack of Baghdad in 1258 and the end of the Abbasid caliphate is taken as the defining turning point in medieval Persian history, an event that falls almost exactly in the middle of the period under review and which to some extent destroys its unity.