The Forty Years' War  was a military conflict fought between the Burmese-speaking Kingdom of Ava and the Mon-speaking Kingdom of Hanthawaddy Pegu. The war was fought during two separate periods: 1385 to 1391, and 1401 to 1424, interrupted by two truces of 1391-1401 and 1403-1408. It was fought primarily in today's Lower Burma and also in Upper Burma, Shan State and Rakhine State. It ended in a stalemate, preserving the independence of Hanthawaddy, and effectively ending Ava's efforts to rebuild the erstwhile Pagan Empire.. Who was the war fought between?