Answer based on context:

Salin turned out to be the last battle of the war. After the successive battlefield defeats, the bickering between the Confederation's two main powers broke out in full force. Sawlon II of Mohnyin, who had only grudgingly agreed to Hkonmaing's takeover of the Ava throne, blamed Hkonmaing for the defeats, and now planned to put Sithu Kyawhtin I of Salin on the Ava throne. In April/May 1545, Sawlon II sent Sithu Kyawhtin with an army , which went on to occupy Sagaing, the city directly across the Irrawaddy from Ava. During the rainy season, c. September 1545, Hkonmaing died, and was succeeded by his son Narapati III. The new king promptly sent a mission to Pegu to secure friendly relations in exchange for his recognition of the new de facto border between the two kingdoms. Tabinshwehti accepted the offer. Toungoo's decisive victory gave the upstart kingdom control of all of central Burma, and cemented its emergence as the largest polity in Burma since the fall of Pagan Empire in 1287. Indeed, "there was once more a king in Burma". In the following years, Ava and Mohnyin-backed Sagaing would be locked in a war until 1551, while an emboldened Toungoo would turn its attention to conquering Arakan in 1545-47, and Siam in 1547-49.

Where did Toungoo turn its attention to conquering first, Arakan or Siam?
Arakan