Under Malik-Shah I (1072–1092) and his vizier Nizam al-Mulk, the Great Seljuk empire reached its zenith — from the Mediterranean to Central Asia — and within weeks of their deaths in 1092 began to break apart.
Malik-Shah inherited at seventeen, defended the throne against his uncle at Kerj Abu Dulaf, and then presided — with Nizam al-Mulk, the Persian vizier who had served his father — over the Seljuk noon: Transoxania to the gates of Egypt, the khutba read for him, as the chroniclers boasted, from Kashgar to Jerusalem and Yemen.
The partnership of Turkish sword and Persian pen built the classical Islamic state — the Nizamiyya madrasas, the reformed "Jalali" calendar computed by Omar Khayyam's commission, the iqta system regularised. In 1092 an Assassin's knife took the vizier and fever (or poison) took the sultan within five weeks, and the edifice cracked along every dynastic seam at once.
Born in 1055, the year his great-uncle Tughril entered Baghdad, Malik-Shah was raised for empire under Nizam al-Mulk's personal direction and named heir over older claimants by Alp Arslan. He was with the army on the Oxus in 1072 when a captured castellan stabbed his father in the sultan's own tent.
The accession was contested as every Seljuk accession would be: his uncle Qavurt of Kirman asserted the steppe principle of seniority and was defeated at Kerj Abu Dulaf in 1073, then strangled with a bowstring — the settlement's grim punctuation, conventionally blamed on the vizier's counsel.
The chroniclers' Malik-Shah is the huntsman-king of the Persian ideal: magnificent, open-handed, a builder of mosques and towers of gazelle horns with equal enthusiasm, pious enough for two pilgrimages and worldly enough for the polo grounds of Isfahan, his beloved capital. Ibn al-Athir's obituary — justice, splendour, and the empire's golden security of roads — is the official portrait, and much of it is verifiable in stone and institution.
The question his reign never had to answer alone was competence apart from Nizam al-Mulk: for twenty years the vizier's administration ran the empire while the sultan campaigned and adjudicated. The final months suggest the answer the empire feared — turned against the aging vizier by the Assassins' whisperers and his own household's factions (his wife Terken Khatun pressing her infant son's claim), Malik-Shah let the old man fall to the murderers on the Baghdad road, and had five weeks to regret it.
The frontiers ran outward in every direction: Transoxania to Kashgar under direct or tributary rule after the campaigns against the Qarakhanids; Syria organised under his brother Tutush; Anatolia — after Manzikert's opening — filling with Turkmen under Suleiman ibn Qutalmish, whose Rum sultanate at Nicaea was a cousin's state, obedient mostly in name. Fatimid Egypt alone held the line of active hostility, and the proto-crusade quarrels over Jerusalem and the pilgrim roads incubated in that standoff.
The interior work outlasted the borders: the Nizamiyya colleges from Baghdad to Nishapur trained the Sunni bureaucratic-clerical class for centuries (al-Ghazali held the Baghdad chair under this reign); the Jalali calendar of 1079 corrected the solar year to a precision Europe would not match for five hundred years; and Nizam al-Mulk's Siyasatnama, written for this sultan, became the mirror-of-princes for the whole Perso-Turkish world. Against it all grew the reign's dark twin — Hasan-i Sabbah's Assassins in Alamut, whose first great victim, in October 1092, was the vizier himself.
The double death of 1092 is one of medieval history's cleanest hinges: within five years the empire was a battlefield of brothers, nephews, and atabegs — and when the First Crusade marched into Syria in 1097, it passed through a Seljuk world too busy fighting itself to combine against the newcomers. Antioch and Jerusalem fell into the crack that opened at Malik-Shah's deathbed.
What endured was the machinery: the madrasa system, the iqta military economy, the sultan-caliph constitutional balance — the framework within which Zengi, Nur ad-Din, and Saladin would eventually reassemble the response to the crusades. The Great Seljuk moment was brief; its institutions governed the region for centuries.