Political unification returned in the mid-16th century, due to the efforts of Taungoo, a former vassal state of Ava. Taungoo's young, ambitious king Tabinshwehti defeated the more powerful Hanthawaddy in the Toungoo–Hanthawaddy War (1534–41). His successor Bayinnaung went on to conquer a vast swath of mainland Southeast Asia including the Shan states, Lan Na, Manipur, Mong Mao, the Ayutthaya Kingdom, Lan Xang and southern Arakan. However, the largest empire in the history of Southeast Asia unravelled soon after Bayinnaung's death in 1581, completely collapsing by 1599. Ayutthaya seized Tenasserim and Lan Na, and Portuguese mercenaries established Portuguese rule at Thanlyin (Syriam).