So, David fought and won against the peoples that surrounded his own nation. Per this chapter alone, He defeated Philistines, Moabites, Edomites, Ammonites, Amalekites & Syrians. Additionally, some of the other nations that he didn't fight became friends with and/or vassals of David and contributed to his power, prestige and wealth. And, within David's court, the leadership was firmly established also. Joab was David's top general, Zadok and Ahimelech were the priests and David's sons (over time) were also given power and position. Among the primary leaders was one named Benaiah? It's unclear to me what exactly his job description was under David's reign, but he was destined for later greatness for sure, especially under Solomon. Incidentally, Benaiah’s name is found 42 times in the Bible.
The overall point of this chapter is that David's kingdom was established. In the areas of foreign and domestic affairs, he was solidly in control. However, it wasn't David's own efforts or expertise that brought about his success, it was God's providential provisions upon him which were the key (II Samuel 8:6 & 14). Not that everything David did was necessarily pretty. Remember, he was fighting wars! We naturally recoil a little when we read that David cut the tendons in the hind legs of Hadadezer's war horses (II Samuel 8:4). And, it may seem a mite odd to read that David "flipped coins" (so-to-speak) to determine which Moabites would survive and which one's would perish (II Samuel 8:2). Yet, in all of this, it was God's hand of blessing upon him that produced his military and political prosperity. Furthermore, if we could examine his physical, familial, financial or spiritual prosperity, we would find God behind all of that as well. God was his preserver.