Pagan's collapse was followed by 250 years of political fragmentation that lasted well into the 16th century. Like the Burmans four centuries earlier, Shan migrants who arrived with the Mongol invasions stayed behind. Several competing Shan States came to dominate the entire northwestern to eastern arc surrounding the Irrawaddy valley. The valley too was beset with petty states until the late 14th century when two sizeable powers, Ava Kingdom and Hanthawaddy Kingdom, emerged. In the west, a politically fragmented Arakan was under competing influences of its stronger neighbours until the Kingdom of Mrauk U unified the Arakan coastline for the first time in 1437.