Because of the time that Hushai bought for David, he had the opportunity to settle down and organize his loyal few. So, he divided his lone battalion into companies and batteries under Joab, Abishai and lttai. They were to go and fight the attacking forces of Absalom, who were led by Amasa (Joab, Abishai and Absalom's cousin). It was more than just another civil war... it was a major family feud; a bloody and bitter conflict that would take the lives of thousands of men. And, in this case the old timers were destined to come out on top.
Now, David offered to go out into the battle with his army, but they would not allow it. He had crossed a line in circumstance (if not in chronology) that made him more valuable back at headquarters than on the battlefield. This point comes for almost every good man who lives long enough. It is time to give advice and leadership and let the grunt work be done by the grunts. The down side of this is that sometimes things don't get done quite the way the old man wants them to be done. And, such was the case for David. He explicitly instructed his captains to fight against Absalom's troops, but not against Absalom himself (II Samuel 18:5). But, that was not to be. When Joab found Absalom hanging inexplicably by his hair from an oak tree in the middle of the battle, he wasted no time striking darts through the wicked prince's heart.