How a Forty-Year Stalemate Between Two Kingdoms Redrew the Map of Medieval Burma
Spanning four decades, the conflict between the Burmese Kingdom of Ava and the Mon Kingdom of Hanthawaddy began as a dynastic power struggle and evolved into a grueling test of endurance. Ending in a stalemate, the war permanently halted Ava's ambitions to restore the ancient, fallen Pagan Kingdom.
The Forty Years' War, also known as the Mon–Burmese War, unfolded in two distinct phases between 1385 and 1424. It was ignited by political instability in the Kingdom of Hanthawaddy following the 1384 rise of King Razadarit. When local governors rebelled against his rule, they invited King Swa Saw Ke of the northern Kingdom of Ava to intervene, triggering an invasion that began in December 1385.
The early years were defined by tactical maneuvering and failed sieges. Ava initially underestimated the resistance, sending armies led by young princes that were thwarted by Pegu’s defensive fortifications along the Sittaung River. Despite Ava’s numerical superiority and naval strength on the Irrawaddy, Razadarit’s forces—led by commanders like Byat Za and Lagun Ein—successfully utilized ambushes and decoys to repel multiple invasions. By 1391, the conflict reached an uneasy status quo, leading to the first of two major truces.
The second phase, beginning in 1401, saw the conflict expand as both kingdoms sought regional dominance. Ava aggressively pursued expansion into the Shan states and Arakan, prompting Pegu to form strategic alliances to check its rival's growth. The war reached a turning point in 1415 when the formidable Avan Crown Prince Minye Kyawswa was killed in battle. His death drained the momentum from both sides, leading to a series of half-hearted campaigns.
The war finally concluded in early 1424 after a diplomatic resolution. Pegu’s Crown Prince Binnya Ran I secured peace by offering his sister, Shin Sawbu, in marriage to Ava’s new king, Thihathu. While the war ended in a stalemate, it fundamentally altered the political landscape of the region. It preserved Hanthawaddy’s independence and effectively signaled the end of Ava’s efforts to reconstruct the legacy of the erstwhile Pagan Kingdom.
Source: Forty Years' War
