In this chapter, the Israelites enter battle with several nations on the outskirts of the promised land.
This chapter begins with a new conflict. Having been denied passage by the Edomites, we are told that "the Canaanite" fought against Israel. The Canaanites are a patchwork collection of tribes that primarily run along the Mediterranean coastline. We are told in Gen 10:19 that the Canaanites extend from Sidon (a city on the coastline, north of the promised land) to Gaza, which is also on the coast in southern Israel, towards Sodom and Gomorrah which are in the plains of the Jordan somewhere, giving them control of most of the promised land into which the Israelites are eventually heading.
These are the descendants of Canaan, son of Ham. They are not closely related to the Israelites, and as we can see their conflict begins very quickly. Notably, Israel is not invading up through the (Canaanite-controlled) coastline; as v. 4 points out, Israel is heading out, around Edom and through the desert, to invade the promised land from the east. So the Canaanites are not defending; they are attacking in some form of a preliminary strike, anticipating the Israelite threat. It goes poorly for them, as the LORD supports Israel and they defeat the Canaanites, destroying "their cities". That is, the cities under the king of Arad; this is only one of the many Canaanite mini-kingdoms.
The people move on, and the complaining resumes. In verse 5, the people complain that there is no food nor water, and then remembering that they are given supernatural manna, say that they dislike the manna. It seems like the LORD simply can't do anything right in their eyes. In response, the LORD punishes them by sending "fiery serpents", with "fiery" referring to either: the pain felt upon being bitten, the red color of inflammation from being bitten, or lastly the red/orange color of the snake itself. I think the epithet is more likely to refer to their poisonous character, because the same sentence refers to "many people of Israel" dying from the bites.
The people realize their mistake and repent; in response, rather than take away the snakes, the LORD instructs Moses to craft a bronze snake and put it up on a standard where it could be seen by everyone, and looking at this bronze image would heal them from the snake venom (if not symptom-free, at least they would live).
This is a peculiar account for a few reasons, but the biggest is that it appears as if the LORD instructed Moses to create an idol, contrary to the command in Ex 20:4. The bronze or copper color of the serpent is likely intended to mimic the "fiery" character of the snakes plaguing the Israelites. Also, there doesn't appear to be any worship or offerings to the snake, which suggests that while it appears to be a graven image, it is not treated as symbolic of any particular deity, and thus is not an idol in the strict sense.
Even if we can (tenuously) conclude that it is not any sort of an idol, there is still a lingering perplexity why this is the form of protection that the LORD crafts for his people. Compared to the plagues of Egypt, for instance, and in that case the LORD dealt with the various plagues of insects/frogs by driving them off or slaying them. In this case, the snakes remain to torment the people, but if they look at a graven image of a snake, then they are healed from death.
It is also unusual that the LORD punishes them with serpents, as this is the first time a serpent has been mentioned since Ex 7 when Aaron and Pharaoh's court magicians turned their staves into snakes. Given what we have read so far, it is possible that the serpents of this story are intended to hearken back to the serpent of Gen 3, which led Eve and Adam into sin. As I discussed then, the snake is a biblical symbol for Satan, but since there are many snakes here it is probable they are meant to be something related to the devil: demons, or possibly sins. It would be fitting for the snakes to symbolize sin here, since they are sent as a punishment for the people sinning against the LORD.
But if that is correct, then it still leaves me wondering why the bronze serpent is put upon a standard and held aloft, and how that would heal the people of the snake bites. With the Passover, we can see clear connections between the blood on the doorways, the sacrificial system of Leviticus, and healing the curse from Gen 3. This chapter also takes us back to Gen 3 by re-using the serpent, and that leads us to the conclusion that the bronze serpent is somehow symbolic for the same healing as the Passover, though in a different way.
The bronze serpent certainly shares the unity of the Passover; in the Passover, each family was to sacrifice a single male lamb. So while there were many lambs slain, the emphasis is on "one" because we are not given a total number that must have been, or were, slain. Here, there is literally one bronze serpent that is held aloft. The Passover emphasizes substitutionary atonement, while this passage emphasizes "looking upon" the serpent to be healed, which doesn't necessarily imply that the "sin" (i.e. poison) is transferred to the serpent, but somehow awareness of the bronze serpent is sufficient to bring about healing.
That said, there is a big difference between the Passover, which must be remembered every year and is a core element of the covenant, and this chapter which is a single, non-repeated incident. For that reason, I don't think it's fair to put these two practices on the same level, because they're not. I only think it's worth mentioning them both because this chapter, by its use of serpents and very unusual symbolism, seems to demand a bit more careful analysis than a lot of the other various plagues and incidents we have seen so far, especially in Numbers (which is primarily a narrative book).
Moving on, the people head out and going around to the east of Moab, and then north into the land of the Amorites. Moab, like Edom, is a kingdom that is blood-related to Israel, because Moab is descended from Lot, the cousin of Abraham. They aren't given much mention here, but we can be confident that Israel will not invade Moab for that reason. On a related note, the Ammonites in this passage are the sons of Ben-Ammi, one of the two sons of Lot and the "brother" of Moab. Israel will also avoid hostilities with Ammon for that reason. There are a few brief poems in here which I think are pretty interesting, but not significant enough to be worth any comments here.
Much more significantly is what happens next, as Israel engages in battle with Sihon and Og, two kings of the Amorites, slays them and kills most of their people. Unlike the Moabites, the Amorites are not related to Jacob or Abraham. Rather, we are told in Gen 10:16 that "the Amorite" is a son of Canaan. The bible (and most historians) treat them as separate groups, listing them separately, but we can hypothesize some sort of familial connection between the Amorites and the Canaanites. Certainly both groups share a deep hostility to Israel, and that results in the battles we see here.
Also, note that the land of the Amorites described here is east of the Jordan, and therefore falls outside of the promised land. That's why Moses is willing to negotiate passage with them rather than attacking them outright. Similar to the Moabites, the Amorites refuse this offer, and probably for the same reasons, fearing both betrayal (that Israel would attack once within the land, gaining a superior strategic position) and theft of water. Since the Amorites are not related to Israel, Moses is willing to engage them in battle, unlike the prior chapter.
This leaves Israel in the interesting position of possessing land outside of the promised land, which had been there objective, and they have every appearance of settling this windfall property. This has some interesting implications both politically and theologically, since it is the promised land that is central to the Abrahamic covenant. What does it mean for some Israelites to live outside of that promised inheritance, and how will the other tribes react? The answer to that we will find later in the story.
For now, I want to emphasize two things. First, we can see that taking these lands was a military necessity as the Israelite tribes wished to move into the promised land. Though they were not promised the lands east of the Jordan, they were forced to conquer them by the former inhabitants. But now that they have conquered these lands, there are unforeseen consequences, as the Israelite nation begins to grapple with their future. This will be a lingering issue from what was an intermediate cause.
Second, these are two of the first serious military engagements by the Israelites. Previously they had battled the Amalekites (Ex 17) and the Canaanites (Num 20), but now they have slain two kings, destroyed their kingdoms, and taken the land for themselves, which is a much more significant action. It also foreshadows the broader conflict as they enter the promised land and take on the many nations therein. We will see these initial conquests mentioned quite a few times as a sort of "pep rally" to boost the Israelite morale, which shows us that however briefly the battles are described, they must have involved significant numbers of combatants or been otherwise noteworthy.