In this chapter, the Ammonites spark a conflict with David, and are defeated along with the Arameans.
The first interesting point in this chapter is the interplay between David, Nahash and Hanun. In verse 2, David asserts that "Nahash showed kindness to me". I did a quick search for references to Nahash, and the only time he is ever mentioned before this is when he gathered up forces to fight a war against Jabesh-Gilead in 1 Samuel 11. So it is entirely unclear what Nahesh has done to "show kindness" to David, though it's possible that this phrase indicates a peace treaty existed between the two countries.
Hanun's suspicion seems warranted, given the long and violent history between Israel and Ammon. As far as I can tell, they have only ever fought wars against each other. There is no recorded interaction between David and Nahash of any positive character. If we trust the narrative here, then it would seem that David must have thought favorably of Nahash and Hanun mistreated David's emissaries. I think that is the author's perspective. However, as mentioned, it is not clear why we should trust David's goodwill if the author did not insist it were so.
This chapter, itself, perpetuates that negative relationship, as Israel takes this offense as yet another justification for war, and the Ammonites raise an army to preemptively attack Israel. Ammon rallies the Arameans as an ally in this conflict, which draws Israel into a larger regional conflict in which they are again victorious.
The second interesting point in this chapter is that it again shows how deeply David depends on Joab and Abishai. This is why Joab was able to get away with murder, because David still needs Joab to lead his armies. It is striking to see that Joab is the commander of the army and not David. David is likely a skilled warrior, and during the early period of Saul's reign, David was commander of Israel's armies. For whatever reason, that is no longer the case, perhaps because of old age or perhaps because of his busy affairs as the head of state.
Verses 4-5 also share some interesting details about what constitutes embarrassment in ancient times. Hanun cuts off half of their garments and half their beards, and David urges his men to wait in Jericho (near the Jordan river, on the outskirts of Israelite territory) until their beards have regrown. There are not many references to beards in the bible, but what we can infer from this passage is that for men, a long beard is a sign of respect and dignity, probably because of their veneration for the wisdom of old age and "elders", and cutting a man's beard forcibly would be an insult for this reason. Cutting off their garments would "expose their nakedness", a form of disgrace that has been reviled in Israelite society ever since the days of Noah (Gen 9:22-23, Exodus 20:26).
Otherwise, we are given few details of the conflict between Israel and the various foreign powers, except to note that Israel defeated all of them and they all made peace with Israel, likely at a high price of tribute or other concessions.