Is that any worse than the biases of contemporary or contemporary-ish reports?
It is not worse, but engaging in revisionism prevents answering the question whether something was justified, or more precisely, why did the people who acted at the time believed justified, or if they did.
If one looks at the European ‘Wars of Religion’ of the 16th and 17th centuries, for example, people thought of them in religious terms, spoke of them in religious terms, reported them in religious terms but they had an awful lot to do with power, territory and profit as well - ideology and greed have had a long acquaintance.
Clearly. France, whose government was at least nominally Catholic, consistently supported the Protestant cause against the Holy Roman Empire, which sought to maintain its structural and religious integrity, although it was negotiable on religious freedom to higher degree than many contemporary states.
It is worthwhile to observe that modern governments act the same way. The United Kingdom believed itself justified in suppressing the Irish rebellions. The USA believed itself justified in supprerssing the Confederate rebellion. Russia believes itself justified in suppressing the Chechyn separatists. Bosnia believes itself justified in suppressing the Orthodox seperatists, and NATO agrees and assists it. Isreal believes itself justified in suppressing Palestinian attempts to secede from its political control. China believes itself justified in suppressing the Tibetans.
One might ask, as a matter of psychology, how many of the Isrealites motives were “pure” when Joshua led the people over the Jordan. Is it realistic to assume that the same people who made the Golden Calf and berated Moses were totally transformed in the span of one generation? How many were motivated by the desire for someone else’s land? Scripture remarks on this, when the Lord says “I have given you cities you have not built, you eat of vinyards that you did not plant.”
As a matter of realpolitik it comes down to Richard Burton’s line in Alexander the Great, when he was asked to whom he left his empire: “To the strongest,” was the reply.
It is easy to identify impure motive and error. It is hard to say when one group has the right to demand -and fight for- self governance and freedom. In the long history of humanity, it has never been definitively answered.