The Seljuk Turks
In the 11th century, a Turcoman tribe called the Seljuks set up a state in Iran, with Isfahan as their capital. The Abbasid Caliph in Baghdad was so taken in by their military prowess, that he sanctioned their leader, Tugrul Bey, with the title "King of the East and West" thus designating the Seljuk warlord as his temporal deputy.
But the Seljuks under Tugrul and his successor, Alp Arslan, were not content with controlling only their piece of the disintegrating Arab empire: recent converts to Islam, they saw themselves as the rightful heirs to the lands conquered during and immediately after the time of the Prophet Muhammed, in particular, the heretical lands of the Levant and Egypt. Indeed, in order to secure their own flanks, Isfahan entered into numerous negotiations with the Byzantine emperors of Constantinople (Istanbul).
But however sedentary and acculturated the Seljuk chieftains had become, the situation on the borderlands between the Seljuks and the Byzantines was anything but peaceful. Turcoman gazis (warriors for the faith, and generally a very motley crew) and Byzantine akritoi (mercenaries) were enrolled as private troops for various Armenian - Byzantine landowners but engaged in private looting themselves. The Seljuks and Byzantines constantly accused each other of bad faith and for breaching the general peace. It was not until the third quarter of the 11th century , when the situation reached a critical point that the Byzantines, under Emperor (or Basileus) Romanus IV Diogenes, decided to preempt the nascent Seljuk power on their eastern frontier and re-conquer Armenia.
Using ancient Harput (modern Elazig) as his base, Diogenes crossed the Euphrates (the classic demarcation of east and west) to confront the Seljuk army on the field of Manzikert (Malazgirt), north of Lake Van in 1071. Although they vastly outnumbered the irregular Turkish horsemen, the Byzantine Christian troops could scarcely have selected a worse venue: the light-riding Turks feigned a retreat, lured the main Byzantine force into a loop, and showered the heat-exhausted Christian host with arrows before closing on three sides with the scimitar. The booty for the victors on "that dreadful day" included the vanquished Diogenes himself.
Remarkably, the Seljuks did not drag the beaten Diogenes back home in victory, but released him for a ransom and cession of Byzantine land, and reentered a period of often uneasy peace with Constantinople (Istanbul) again. Indeed, the two forces actually stood together against the Mongol invasion of the 13th century. But it was a vain defense as neither Christian nor Muslim were spared the sword as the Mongol hordes rolled across the steppe into Anatolia.
The reigns of Alp Arslan and his son, Malik Shah, were the most glorious years of the great Seljuks of Isfahan; the death of the latter marked the decline of the great Seljuks and by 1192 the dynasty ended in the same obscurity with which it had begun, unable to cope with the pressures from the Crusaders, the caliph and new Turcoman clans arriving from the east due to the increasing power of the Mongols, who were soon to erupt from the deepest recesses of Central Asia to sack much of the known world before returning just as quickly to the frontiers of China.
Following the decline of the great Seljuks and the onslaught of the Mongols, lesser Seljuk clans established their own principalities throughout Anatolia and made the small Christian states in the area their vassals. Through inter-marriage, they greatly facilitated the cultural syncretism of the area. The presence of so many petty Muslim states in east and central Anatolia explain the abundance of Seljuk architecture in modern Turkey, with some of the best examples of this so-called "poetry in stone" to be seen in Erzurum, Divrigi, Sivas and Konya. Of these, Konya is perhaps the most impressive. It was where the Sufi mystic, Celaleddin Rumi, (Mevlana or "our master") graced the court of Alaaddin Keykubat I, the Sultan of Rum and initiated the peculiar whirling dervish ceremony in an effort to seek spiritual union with the Creator himself. The cultural effervescence at Konya, however, met with the same abrupt and unhappy end as the others at the hands of the indiscriminate Mongol hordes of the descendants of Cengiz Khan.
Just as they had dealt the Byzantines a decisive blow at Manzikert (Malazgirt) two centuries before, the now settled Seljuks could not resist the most recent wave of nomads from the steppe. On June 26, 1243, despite Byzantine auxiliaries sent by the Seljuk Sultan's "ally" in Constantinople (Istanbul), the once mighty Seljuk army was utterly routed at Köse Dagi outside the quintessentially Seljuk city of Sivas. The remaining Turkish clans scattered westwards before being further defeated by the Mongols, until they had no choice but to finally accept their role as mere vassals in the greater scheme of things. But no sooner had the Mongol tide surged over the region than it withdrew once more, leaving behind several unimportant mini-states led by petty chieftains who might well have remained utterly obscure but for one of their number on the fringe of the Byzantine state: Osman, the son of Ertugrul, the horseman destined to found a new empire that stretched from Morocco in the west to Iran in the east, and from the Yemen in the south to the Crimea in the north. This new empire was called as Ottomans.