In this chapter, Israel defends Gibeon against five Amorite kings with miraculous assistance, and destroys many cities in a counterattack.
The OT is filled with really strange miracles, and this chapter has a couple of them. I think these miracles are especially peculiar because the rest of the chapter is so consistent with the stories of the Pentateuch and Joshua.
As with Gibeon, Jerusalem, Hebron, Lachish, etc. are all located in eastern and central Israel, in what verse 6 calls "the hill country" and they are understandably threatened by Gibeon's negotiated peace with Israel. What they fear is a classic divide and conquer strategy by Israel, allying with some of the Amorites and destroying the others, fragmenting their resistance. We have been told a bunch of times that the Canaanites are terrified of the Israelites, and they know they are fighting for survival. They are attacking Gibeon 1) as a warning to the other Amorites not to make peace with Israel and 2) because they probably also fear that the Gibeonites will fight alongside Israel's armies. My guess is that they're attacking Gibeon to destroy them before they join together their forces with Israel.
The Gibeonites call on Israel (as their new lord) to defend them and interestingly, Israel agrees to do so and sends their armies on a hastened overnight march. I can't help but wonder if the Israelites saw this as an unexpected solution to their Gibeonite "problem", so to speak. I mean, they had sworn not to harm the Gibeonites but were commanded by the LORD to wipe them out. It seems convenient that the other Amorites would come along and prepare to destroy Gibeon; Israel had to simply do nothing and the "problem" would be "fixed". I can only guess that assisting the Gibeonites was part of the sworn covenant between them, so Israel was probably bound to help them as well. That would probably explain why the Gibeonites expect assistant and call themselves Israel's "servants".
Well for whatever reason, Joshua and Israel marches overnight to help Gibeon and the battle goes well; very well. It begins with the LORD "confounding" their enemies, resulting in a "great slaughter", then as their foes are retreating it escalates to the LORD throwing "large stones from heaven" upon their enemies, slaying more men with hailstones than were killed by the sword. As if that weren't enough, it ends with the LORD stopping the movement of the sun in the sky for about 24 hours. This is very useful for a victorious army because it's easier to chase and hunt your enemies in daylight than at night. What this means is that the victory Israel had won through the "confounding" and the hailstones was extended by giving them more daylight to pursue the Amorites, wiping out the survivors.
These three miracles, all occurring in the course of a single battle, seem to get more bizarre and extravagant as they go along. I mean, "confounding" your enemies is one thing, and raining hailstones big enough that they can kill people is more unusual, but then stopping the sun in the sky for 24 hours ..... that doesn't happen very often. Verse 14 agrees, saying nothing like it has ever happened before or since.
Remember when I said two chapters ago that the LORD is creative and loves to do new and different things? In conquering Jericho he made the walls fall down on the seventh day; in this chapter he stops the sun while the Israelites kill retreating Amorite soldiers.
In verse 13, the author of Joshua asks "is it not written in the book of Jashar?" This book is mentioned later in 2 Samuel 1:18, but there are no extant copies or other quotations, so we don't actually know what was written in the book of Jashar other than these two references. It is one of the many lost books of the ancient world about which we know nothing.
After destroying the armies of these five cities, Joshua also kills the five kings with a triumphal display, encouraging his men to humiliate their enemies by stepping on their necks. Like many other things in the book of Joshua, this is designed to increase the morale of his troops. As with the king of Ai, Joshua takes down their bodies before nightfall.
Since the armies of these cities were mostly annihilated, the cities themselves were left defenseless. Joshua systematically destroys all of them, slaying every man, woman and child, but keeping the livestock and property as the spoils of wars. Destroying the other cities took divine assistance, but with the defenders already slain when they attacked Gibeon, there would have been little resistance against the Israelites.
I don't have much else to say. The miracles are really peculiar, but the rest of this chapter fits in closely with the narrative of Joshua. Israel is now systematically destroying the Amorites city by city and nobody is able to resist them now that the LORD is fighting on their behalf.