The siege of Urganch of 1379 was the key victory during Tamerlane's fourth war in Khwarezm, and saw the city fall after a siege of three months.
Khwarezm (or Khorezm or any one of a dozen other spellings) was located downstream from Transoxiana, around the delta of the Amu Darya River. The area had once been the centre of a major empire, but that had been destroyed by Genghis Khan, and Khwarezm had become part of the territory of the Golden Horde. This ended at the start of the 1360s when Husayn Sufi seized power. He died soon after Tamerlane's first expedition into the area, at the start of the 1370s, and was succeeded by his brother Yusef Sufi. Yusef came to terms with Tamerlane and agreed to provide his brother's daughter as a bride for Tamerlane's son. A second expedition followed when the bride was not forthcoming, and the marriage quickly took place. A third expedition was very short-lived, but indicated that Yusef was not willing to act as Tamerlane's vassal.
During 1378, while Tamerlane was distracted by the affairs of the Golden Horde, Yusef raided Bokhara. When Tamerlane sent an ambassador to Yusef's court he was seized, as was a messenger sent to protest about the first seizure.
Early in 1379 Tamerlane gathered his army and invaded Khorezm, blockading the capital city of Urganch. At the same time parties were sent out to pillage the rest of the country.
Yusef Sufi was trapped inside Urganch, from where he sent a letter to Tamerlane offering to fight a duel to decide the outcome of the war, presumably not expecting the partly-crippled Tamerlane, who was by now in his mid-40s, to accept the challenge. Yusef had badly misjudged his opponent. Tamerlane donned his armour and rode up to the city walls (after overcoming the resistance of his own Amirs). Hardly unsurprisingly Yusef lost his nerve (if he had ever genuinely intended to come out to fight). After staying outside the walls for some time Tamerlane returned to his camp, having won a moral victory over his opponent.
Soon after this incident a large part of the garrison, under the command of a general called Hadgi, sallied out from the besieged city. They were intercepted by a force led by Tamerlane's second son Omar Shaykh, and a battle began that lasted until nightfall, only ending when the defenders returned behind the walls.
After this fight Tamerlane ordered his men to begin the siege-proper. Battering rams and catapults were set at work, and Yusef's palace was so badly damaged that he was forced to seek shelter elsewhere. This phase of the siege lasted for three months and fifteen days, and towards the end of this period Yusef sickened and died.
After the three-month bombardment Tamerlane's men had created a number of breaches in the walls of Urganch, and at last he ordered an assault on the city. Despite a vigorous defence the city soon fell to the attackers. The city was plundered, and all of the civil public buildings destroyed while the religious leaders and learned men were ordered to move to Kesh, Tamerlane's birthplace.
Urganch would soon suffer a worse fate. In the late 1380s the new ruler of Khwarezm allied himself with Tamerlane's most persistent enemy, Toktamish, and took part in raids into Transoxiana. This triggered Tamerlane's fifth expedition into Kkwarezm, at the end of which Urganch was razed to the ground and barley sown on the site.