The warfare in the Book of Joshua is thought of by some Christians as unacceptable because of its brutality.
In his book The God I Don't Understand Dr Christopher Wright says the following.
"The kind of warfare described in the conquest stories should, first of all, not be called 'holy war' (a term never used in the Bible). It is called 'a war of Yahweh'. That is, it was a war in which the God of the Israelites won the victory over their enemies." (page 87)
"Within that context, the concept of herem (or 'ban') was applied. This meant the total dedication of all that was being attacked - human, animal, material - to God himself. In a battle or war in which herem was declared, there was to be no material profit for the Israelites, since no plunder was allowed. However, the rules of herem varied, as the Old Testament narratives show. Sometimes women and children were spared (Num. 31:7-12, 17-18; Deut. 20:13-14; 21:10-14); sometime cattle could be kept (Deut. 2:34-35). But in the cases of nations living within the land of Canaan itself, the general rule was total destruction." (pages 87-88)
"Now we need to know that Israel's practice of herem was not in itself unique. Texts from other nations at the time show that such total destruction in war was practised, or at any rate proudly claimed, elsewhere. But we must also recognize that the language of warfare had a conventional rhetoric that liked to make absolute and universal claims about total victory and completely wiping out the enemy. Such rhetoric often exceeded reality on the ground." (page 88)
"Admittedly this does not remove the problem since the reality was still horrible at any level. But it enables us to allow for the fact that descriptions of the destruction of 'everything that lives and breathes' were not necessarily intended literally. Even in the Old Testament itself this phenomenon is recognized and accepted. So, for example, we read in the book of Joshua that all the land was captured, all the kings ere defeated, all the people without survivors (such as Rahab) were destroyed (e.g. Josh. 10:40-42, 11:16-20). But this must have been intended as rhetorical exaggeration, for the book of Judges (whose final editor was undoubtedly aware of these accounts in Joshua) sees no contradiction in telling us that the process of subduing the inhabitants of the land was far from complete and went on for considerable time, and that many of the original nations continued to live alongside the Israelites. The key military centres - the small fortified cities of the petty Canaanite kingdoms - were wiped out. But clearly not all the people, or anything like all the people, had in actual fact been destroyed by Joshua." (page 88)
"Even in the Old Testament itself, then, rhetorical generalization is recognized for what it is. So when we are reading some of the more graphic descriptions, either of what was commanded to be done or of what was recorded as accomplished, we need to allow for this rhetorical element. This is not to accuse the biblical writers of falsehood, but to recognize the literary conventions of writing about warfare." (page 88)