Bayezid I "the Thunderbolt" (1389–1402) crushed the crusade of Nicopolis and nearly took Constantinople — before Timur destroyed him at Ankara and the captive sultan died in a conqueror's baggage train.
Bayezid took the throne on the field of Kosovo in 1389 — inaugurating, with his brother's immediate strangling, the Ottoman law of fratricide — and earned his byname by campaigning at storm-speed between two continents: the Anatolian beyliks annexed in a single sweep, Bulgaria extinguished, Constantinople blockaded for six years.
At Nicopolis in 1396 he destroyed the last great Western crusade. At Ankara in 1402 Timur destroyed him: the Anatolian levies deserted to their old emirs, the Serbian vassals died loyal, and the sultan was run down and caged — figuratively at least. His empire dissolved into a decade of civil war among his sons, and Constantinople breathed for another half-century.
Born about 1360 to Murad I, Bayezid earned the name Yıldırım — Thunderbolt — as a prince commanding on the Anatolian frontier. At Kosovo in June 1389 he led the counter-stroke that decided the battle; when his father was killed, the commanders proclaimed Bayezid on the field, and his elder brother Yakub, still commanding a victorious wing, was strangled before he learned why he had been summoned.
The succession method he improvised became dynastic law — codified under his great-grandson Mehmed II — and its logic was his reign's: the Ottoman state would not fragment as every Turkish polity before it had, whatever the cost in brothers.
The sources — Ottoman chronicles, Byzantine historians, the crusade's survivors like Schiltberger — agree on the velocity and the violence: a commander of terrifying speed and personal ferocity, first into the assault, capable after Nicopolis of watching thousands of prisoners beheaded through a morning. They agree too on the appetites the pious chroniclers deplored — the wine, the splendour, the grand vizier's fiscal rapacity tolerated — an un-ghazi magnificence learned partly from the Serbian princess Olivera he loved among his wives.
Contradiction ran through him: the blockader of Constantinople who was also the first sultan to imagine himself its Roman successor, demanding from the caliph in Cairo the title Sultan of Rum; the destroyer of the beyliks who filled his court with their poets. Ottoman memory, shaped by the catastrophe, made him the exemplum of hubris — the Thunderbolt who mocked Timur's letters and lost God's favour with the battle.
The decade of conquests ran on two fronts. In Anatolia he did what no ghazi propriety allowed — swallowed the Muslim beyliks whole (Aydın, Saruhan, Menteşe, Germiyan, finally Karaman) — and in Europe he ended Tarnovo Bulgaria (1393), took Thessaly, and hung a six-year blockade on Constantinople from the new castle of Anadoluhisarı. The relief crusade — Hungarian, French, Burgundian chivalry, the largest since Acre — he annihilated at Nicopolis on 25 September 1396, the French charge breaking itself exactly as at Crécy, on stakes and disciplined infantry with the sultan's counterattack behind.
The eastern reckoning came when the beyliks' dispossessed emirs reached Timur's court. The two empires' letters grew mutually unforgivable, and on 28 July 1402 at Ankara, Timur's host — with elephants, and with the beylik contingents in Bayezid's own army primed to defect — enveloped the exhausted Ottoman force. The Serbs under Stefan Lazarević charged three times to break the ring; the Thunderbolt fought with an axe until his horse fell. He died at Akşehir on 8 March 1403, in captivity that Timur made courteous after the first display, while his sons' civil war — the Interregnum — dismembered everything behind him.
Ankara nearly ended the Ottoman project: the beyliks resurrected, the European vassals loosed, the capital-in-waiting reprieved — Constantinople's fall postponed fifty-one years precisely by Bayezid's defeat. That the state reassembled at all, under Mehmed I by 1413, proved the institutions stronger than the catastrophe; the lesson Ottoman government drew — never again a single battle with everything staked — shaped the careful imperialism of the next two reigns.
His monuments survived him in Bursa — the Ulu Cami and his own complex — and so did the cautionary legend: the caged sultan became a fixture of both Ottoman moralising and European drama, Marlowe's Tamburlaine giving the story its most famous stage.