It does not. There was never an intention in the Bible, or from God's direction, to kill all the Canaanites. The express purpose was to drive them from the land (Ex. 23.30-31; Dt. 7.17; 9.4-5; 12.29; Josh. 13.6, et al.), not kill them. Even during the Conquest, Israel burned only 3 cities: Jericho, Ai, and Hazor. The other cities were taken over. There is no evidence the population was slaughtered; the evidence actually indicates otherwise. God was more concerned about the destruction of the Canaanite religion and idols than Canaanite peoples than of killing the people.

For example, look at Deuteronomy 7.2-3 (same book). Verse 2 says "kill them all!" But then verse 3 says not to intermarry with them. But how can you intermarry if they are ALL DEAD? You can't. This shows there was no genocide. They wouldn’t be around to marry with if they were all dead.

For further explanation, the term [i]cherem[/i], usually translated as "kill them all," actually means "remove from human use." In various places in the Old Testament it refers to...

[list]1. Inanimate objects (including plots of land): plunder (Josh. 6.17), metal objects (Josh. 6.19, 24), a field (Lev. 27.21). These things are assigned to the divine realm. It has nothing to do with killing.
2. Living Individuals (people or animals). It is implied in Josh 6.17 & 8.2; Lev. 27.28. It has nothing to do with killing.
3. Abstractions representing communities of people. The nation of Israel refers to the abstract identity of the community, not to each and every individual Israelite. The same is true of nations who inhabit the land. If [i]cherem[/i] means “remove from use,” then removing an identity from use depends on what identity is used for. Essentially it is the equivalent of disbanding an organization. It is not disposing of the members, but disposing of enough of the organization so that there is no longer any identity as members.
4. Cities. They prohibited all human activity at the site. [i]Cherem[/i] cannot and does not mean “destroy” because apart from Jericho, Ai, and Hazor, no [i]cherem[/i] city was destroyed. The city was removed from use. They drove them out. After that, YHWH leased the land and the cities back to Israel. Because the land is [i]cherem[/i], Israel cannot make use of it for herself, but it belongs to YHWH, and so YHWH can do whatever he wants with it. What He chooses to do with His land is to allow Israel to use it, provisionally on Israel’s fidelity to the covenant.[/list]

The chapter is quite clear in making distinction between nations where spoils were allowed to be taken and those where everything was to be killed. "Kill them all" means "kill them all" here.

The "ban," so to speak, is more of a cultural misunderstanding. In their culture, "Kill them all!" was warfare rhetoric for "win a decisive victory." The Israelites, and even most of the ancient armies and wars, were not "killing them all." The universal language was also typical of judgment by the gods, so they would again have the rhetoric of universality when they were fighting a war they presumed was of divine mandate. In other words, the Israelites were not killing every man, woman, child, and cattle. To take it even further, the Hebrew term [i]cherem[/i] was really a term that means "remove from human use" rather than "kill them all." Sometimes things were killed, yes, but at other times they were set aside, dedicated, or repurposed. It's simply false to assume that God was commanding genocide. There's plenty of evidence from the ancient world as well as from the Bible that was not the case.

I am specifically referring to the “ban” in which every woman, child, cattle was to be destroyed

Keil & Delitzsch write: "The instructions in this chapter have reference to the wars that Israel might wage in the future against non-Canaanitish nations (vv. 15ff.), and enjoin it as a duty upon the people of God to spare as much as possible lives of their own soldiers and also of their enemies."

Richard Elliott Friedman says, "The instructions in this chapter distinguish between the original conquered cities and all other wars. In future wars Israel must offer the other side a chance to make peace and become tributary first—no genocide, no slavery, no land grab. Even the females from a city that declines to make peace can’t be treated as slaves (14)."

Maybe you could be more specific about what your question is. After the conquest, Israel didn't fight any wars of conquest. They were to be content with the land God gave them. King David finished the conquest in taking the whole land of Israel. Between those times, and after David, Israel only fought wars of defense and self-protection, no wars of offense or land-grabs.

What are you thinking they need to justify?

How do I as a Christian justify that God commanded Joshua to kill in Deuteronomy 20?