In the spring of the year, the time when kings go out to battle, David sent Joab, and his servants with him, and all Israel. And they ravaged the Ammonites and besieged Rabbah. But David remained at Jerusalem (2 Sam 11:1)
The verse dates the incident to “the spring of the year, the time when kings go out to battle”. In the ancient near east wars would not (willingly) have been fought in winter. The longer days and better weather of spring were better conditions for going to war. So the phrase “the time when kings go out to battle” is not a moral imperative (e.g. “kings should go out to battle in spring”) but simply a description of practical warfare.
The fact that David sent a general (i.e. Joab) to battle, rather than going himself, is not necessarily unusual for kings of the ancient near east. It might be considered unusual for David since he had often led his forces into battle.
The telling line is at the end of the verse, “but David remained at Jerusalem”, which seems to have been added by the writer because he considered it to be unusual. It is also interesting to note Uriah’s words in verse 11, where he refuses to go home whilst Joab, the army and the ark are camped out on the battlefield. This suggestive of where the king’s proper place should have been.
In summary, then, there are indications within the chapter that David should have led the battle, rather than remaining at home.