The Promises of Exodus
The conquest of Canaan in the Book of Joshua cannot be evaluated without also taking into account the background story in the Book of Exodus. The Israelites only arrived at the borders of Canaan because they had been liberated from slavery in Egypt and promised a new land where they could be free. This story has been incredibly inspirational for liberation movements among the poor and oppressed for millennia.
But many of those liberation movements only focus on the first part of this story, the liberation from slavery in the Book of Exodus. Some focus also on the middle of the story, the wandering in the Wilderness that occurs in Leviticus, Numbers, and Deuteronomy. Too few pay enough attention the end of the story in Joshua — or, if they do, they only pay attention to the promise of a land of milk and honey while completely ignoring the fact that this land is already occupied.
From Liberation to Conquest to Genocide
When you promise people liberation you cannot inspire them by merely promising that they will lose their chains; instead, you must also promise them some better future and some greater goal to work towards.
Thus the promise of liberation in Exodus can only exist in the context of the promise of a new home in Canaan and this, in turn, only exists in the context of the assumption that the current inhabitants of Canaan will lose their homes.
Thus liberation for the Israelites depended on expulsion or extermination for the Canaanites. They fled oppression with a destination that automatically included becoming oppressors. They fled enslavement with a destination that automatically included becoming enslavers — and worse, committing genocide. The roles of expulsion and extermination are thus necessary ingredients; they are not merely there for narrative purposes.
What's more, the Israelites arguably created a new state that imitated many of the features of the state which they fled. They didn't just imitate the oppression and enslavement of outsiders, they also ended up imitating the structures of monarchy, priesthood, and authoritarian bureaucracy which eventually became instruments for oppressing themselves. In trying to create a new society, they ended up recreating the old one.
Even if modern liberation movements do not focus much on this part of the story, some movements do — some make explicit use of the idea that indigenous groups need to be driven out so that someone else can make a new home for themselves and do so with the Book of Joshua as a model. South African Boers used it to justify the taking of African lands. Puritans in New England used it to justify dispossessing and killing Native Americans.
And, unsurprisingly, modern day Israelis have used it as inspiration for their modern-day military activities.
The Book of Joshua and Modern Israel
David Ben-Gurion and other early leaders of Israel, for example, saw themselves as repeating Joshua's conquest of Canaan and David's creation of a united Israelite kingdom. Their current conflicts and decisions have been understood in light of ancient stories of military conquest in the same places. Although they were trying to create a new nation along modern lines, some of the materials they used were ancient monarchic and theocratic models.
The theocratic model has survived in the ideology of ultra-Orthodox Jews who are eager for conflict with the Palestinians and they, in turn, have influenced devout Jews throughout Israel with an extremist, militaristic theology. Israeli soldiers invading Lebanon, for example, were exhorted to follow in Joshua's footsteps — advice which implicitly, if not explicitly, exhorted soldiers to engage in the extermination of non-Jews.
Maps of Lebanon used by the military showed towns with names found in the Book of Joshua instead of their correct Lebanese names. Beirut, to cite just one example, was named Be’erot. This act implicitly asserted Israeli claim to all of Lebanon — a claim which, in the Book of Joshua, came with genocide. That it was human beings who needed to be exterminated was justified by the belief that Satan was in control of land belonging to the Jews and those who lived there were little more than agents of Satan. The land has to be purified from its defilement at the hands of idolaters.