In this chapter, we learn about the first three judges who save Israel from their troubles.
The beginning of this chapter, verses 1-6, are a continuation of the last chapter. It continues with the theme of "the LORD left nations in Canaan to test Israel". What's a bit different is that in this chapter, not only are the Canaanite tribes left alive to test the hearts of Israel, whether they would stay true to the LORD, but also to test them militarily, for those generations which "had not experienced any of the wars of Canaan". I don't have any particular opinion about this, I think it mostly explains itself.
In verses 5-6 we have the intermarriage that was strictly prohibited by the law of Moses, and in verses 7-8 we have the direct outcome of that, which is Israel doing evil things and then getting punished for it. This is all straightforward and implied by the Judges Cycle.
The interesting part begins in verse 9 when we start to read about the judges. The first judge is Othniel, who was the guy that married Achsah after leading the assault on Debir (Joshua 15). Beginning with this story, we start seeing what nations rise up against Israel. Interestingly, it isn't really the Canaanites; Israel has the various Amorite and Canaanite tribes mostly under their control, and by intermarriage they become political allies to these tribes. This is interesting because v. 2 tells us that they remained in the last so that Israel could experience wars against them, but it seems like the larger conflicts are against their neighbors from outside the promised land, but it's all because "the anger of the LORD" being kindled.
Their first oppressor is an Aramean king, who Othniel overthrows, and then after that it is an alliance of Moabites, Ammonites and Amalekites. The Moabites and Ammonites are two nations that descend from Lot's two sons, Moab and Ben-Ammi (Gen 19), and the LORD prohibited Israel from fighting them because they were related. However, the Moabites and Ammonites responded with hostility, prohibiting the Israelites from traveling through their land to the promised land. Now, these two nations have allied together with the Amalekites (who attacked Israel pre-emptively in Ex 17) and they subjugate Israel.
The Amalekites have always been hostile to Israel, but the Moabites and Ammonites share a family tie to Abraham so by tradition and honor they should not be fighting against Israel, yet here we are. This probably reflects badly on them, but of course it reflects worse on the Israelites who sinned to bring this upon themselves in the first place.
The story after this one tells us in a single verse that the Philistines are also fighting against Israel and possibly oppressing them, such that Shamgar needed to "save Israel" in the most outlandish way possible, killing 600 people with a tool that hardly even constitutes a weapon at all.
The longest story is about Ehud, who kills the king of Moab. This is a pretty funny story (seen from a certain perspective) that this king was so fat that the sword could be plunged, blade hilt and all, straight into his body and left there. After Ehud locks the door and leaves, Eglon's servants think he's using the bathroom and wait outside "to the point of embarrassment". It certainly presents some comical images, although the reality of the situation is pretty grim. After Eglon is killed, the Israelites seize the fords of the Jordan which would have allowed the Moabites to flee back to their own country, and they killed every Moabite who tried to cross. Remember that Moab and Ammon are both nations to the south and east of Israel, and although some of them probably would have crossed south through the Negev, the fords of the Jordan are closer and easier, so that's probably why they tried to flee that way, and also why Israel tried to capture the fords to prevent Moab's escape.
Anyway, I think this story is mostly self-descriptive; I don't see much that I need to comment on for my readers to understand what is happening. I think the main thing here that isn't explained is the international politics between Moab, Ammon and Israel. I referenced some of this above. In spite of how they should be allies, Moab and Ammon are joining together to oppress Israel, and Israel retaliates by striking down 10,000 Moabite men. This is the beginning of a long, fractious relationship between these two countries, and things will be no better between Israel and Ammon. In Ex 17 the LORD commanded Israel to destroy the Amalekites, so their emnity should not surprise us either now or in the future.