The Forty Years' War begins in 1385…
1384 CE to 1395 CE
The Forty Years' War begins in 1385 between the Burmese Kingdom of Ava and the Mon Kingdom of Hanthawaddy Pegu fought primarily in today's Lower Burma and also in Upper Burma, the Shan States and Rakhine State.
A truce of 1391 will stay in effect until 1404.