In Poul Anderson's History of Technic Civilization, the Terran Empire is right to prepare for war against barbarians and Merseians but the Avalonians and Freeholders are also right to prepare for war against the Terran Empire and what of the Ansans, Aeneans and Braeans who also resist Imperial annexation albeit unsuccessfully? The Braean case is particularly unjust.
Anderson's Technic History may be unique among future histories in presenting this degree of historical ambiguity. In my formative years, ambiguity was rare and shocking. In juvenile historical fiction, the English were good guys when fighting the French but became bad guys when fighting the Scots.
In three works by SM Stirling, the evil of the cannibal Russians, the slave-owning Draka and the neo-feudal Protectorate is unequivocal. There will never be a narrative in which their enemies are wrong to oppose them.