In the early 16th century, the present-day Myanmar (Burma) comprised several small kingdoms.
The Ava Kingdom, the principal power in Upper Burma in the 14th and 15th centuries, had been fighting a long losing war against an alliance of its former vassal states: the Confederation of Shan States and Prome (Pyay).
Another former vassal state Toungoo (Taungoo), hemmed in by the Bago Yoma range in the west and the Shan Hills in the east, stayed out of the internecine warfare raging in Upper Burma.
As the only peaceful state in Upper Burma, Toungoo had received a steady stream of refugees.

Then, in 1527, the Confederation finally defeated Ava, and in 1532, its erstwhile ally Prome.
Its paramount leader Sawlon I of Mohnyin had now reunited most of Upper Burma and cis-Salween Shan states for the first time since 1480.
Toungoo, the only remaining holdout, was his "obvious next target."
Fortunately for Toungoo, Sawlon I was assassinated in 1533 on his return trip from Prome, and his son and successor Thohanbwa, based out of Ava, was not accepted as the first among equals by other saophas (chiefs) of the Confederation.
Without a strong leader, the Confederation suddenly ceased to be a unified force capable of combined action.
In the early 16th century, the present-day Myanmar (Burma) comprised several small kingdoms. The Ava Kingdom, the principal power in Upper Burma in the 14th and 15th centuries, had been fighting a long losing war against an alliance of its former vassal states: the Confederation of Shan States and Prome (Pyay). Another former vassal state Toungoo (Taungoo), hemmed in by the Bago Yoma range in the west and the Shan Hills in the east, stayed out of the internecine warfare raging in Upper Burma. As the only peaceful state in Upper Burma, Toungoo had received a steady stream of refugees.
Then, in 1527, the Confederation finally defeated Ava, and in 1532, its erstwhile ally Prome. Its paramount leader Sawlon I of Mohnyin had now reunited most of Upper Burma and cis-Salween Shan states for the first time since 1480. Toungoo, the only remaining holdout, was his "obvious next target." Fortunately for Toungoo, Sawlon I was assassinated in 1533 on his return trip from Prome, and his son and successor Thohanbwa, based out of Ava, was not accepted as the first among equals by other saophas (chiefs) of the Confederation. Without a strong leader, the Confederation suddenly ceased to be a unified force capable of combined action.
Toungoo–Ava War (1538–1545)