The Hittite Kingdom reached its zenith during the reign of Suppiluliuma I (~1350–1322). The kingdom of Mitanni, wracked by civil war, was unable to withstand the Hittite onslaught. Suppiluliuma swiftly attacked the Mitanni heartland, capturing and plundering the Mitanni capital of Washshuganni. He then turned west, recrossed the Euphrates and captured all the Syrian kingdoms which were vassals to the Mitanni, including Aleppo, Mukish, Niya, Qatna, Upi (Upina), and Kadesh. Other kingdoms such as Ugarit and Amurru (an Egyptian vassal) voluntarily became vassal states of the Hittites. 