Chapter eleven ends with this note: So Joshua took this entire land. . . .He captured all their kings and struck them down, putting them to death. 18 Joshua waged war against all these kings for a long time. 19 Except for the Hivites living in Gibeon, not one city made a treaty of peace with the Israelites, who took them all in battle. 20 For it was the Lord himself who hardened their hearts to wage war against Israel, so that he might destroy them totally, exterminating them without mercy, as the Lord had commanded Moses.
Chapter 12 summarizes the conquest with the list of kings conquered.
The kings on the east of the Jordan were regional kings, and you should note that they were overcome in two battles. The kings on the west side of the Jordan were more town leaders than major kings, but they were the legitimate rulers of their area. It took much longer to conquer them.
Do not miss this very important point: God would not allow these kings to make a treaty of peace with Israel. He never intended His people be polluted by the materialism, sensuality, and idolatry of these nations. Furthermore, though it might have been an option to just leave the land, it does not appear God was going to allow this either. He was determined to wipe out these people who had so desecrated His earth with their ways.
Christians are not called to kill worldly people. We ARE called to reach them with the gospel. We may not, however, make spiritual treaties with them. Their ways are not alternative lifestyles for us, and their beliefs are not equal to ours. We must never behave toward them in anything but a Christ-like manner, but that must never include accepting their ways as viable for our own. We must live lives of peace in the world, but never be “at peace” with it.