The Toungoo–Ava War (1538–1545) was a military conflict that took place in present-day Lower and Central Burma (Myanmar) between the Toungoo Dynasty, and the Ava-led Confederation of Shan States, Hanthawaddy Pegu and Arakan (Mrauk-U). 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.
The war began in 1538 when Ava through its vassal Prome threw its support behind Pegu in the four-year-old war between Toungoo and Pegu. After its troops broke the siege of Prome in 1539, Ava got its Confederation allies agreed to prepare for war, and formed an alliance with Arakan. But the loose alliance crucially failed to open a second front during the seven dry-season months of 1540–41 when Toungoo was struggling to conquer Martaban (Mottama).
The allies were initially unprepared when Toungoo forces renewed the war against Prome in November 1541. Due to poor coordination, the armies of the Ava-led Confederation and Arakan were driven back by better organized Toungoo forces in April 1542, after which the Arakanese navy, which had already taken two key Irrawaddy delta ports, retreated. Prome surrendered a month later. The war then entered an 18-month hiatus during which Arakan left the alliance, and Ava underwent a contentious leadership change. In December 1543, the largest army and naval forces of Ava and the Confederation came down to retake Prome. But Toungoo forces, which had now enlisted foreign mercenaries and firearms, not only drove back the numerically superior invasion force but also took over all of Central Burma up to Pagan (Bagan) by April 1544. In the following dry season, a small Ava army raided down to Salin but was destroyed by larger Toungoo forces.
The successive defeats brought the long simmering disagreements between Ava and Mohnyin of the Confederation to the forefront. Faced with a serious Mohnyin-backed rebellion, Ava in 1545 sought and agreed to a peace treaty with Toungoo in which Ava formally ceded all of Central Burma between Pagan and Prome. Ava would be beset by the rebellion for the next six years while an emboldened Toungoo would turn its attention to conquering Arakan in 1545–47, and Siam in 1547–49.