The city is a sovereign city-state, Everyone in Imago is burning the place like it's not their city or like they have some where else to go after it's gone. they were definitely hired by one of the surrounding countries. The city has a port, large swaths of land and if one country tried taking the city by military force another country would most likely intervene in some way, but if the city self destructs and can no longer take care of itself, a neighboring country could come in under humanitarian pretense.