Commentary for II Samuel 8:15

Dovid rendered judgement… Yoav…was [commander] over the army. Dovid caused Yoav to be successful [with his command] over the army because he rendered judgment and righteousness and Yoav caused Dovid to render judgment and righteousness because he [Dovid] judged and Yoav policed and enforced on the basis [of the judgment].¹⁰Rashi is learning that the proximity of the verse discussing Dovid’s judgment of the people to the next verse discussing Yoav’s commanding of the army shows that these two phenomena were related. Other prominent leaders are being discussed in the subsequent verses but Yoav is mentioned first immediately after king Dovid. Another [reason is that] because Yoav busied himself with the battles, Dovid was not preoccupied by them and his heart was [able to be] open to judge righteously.