2 Samuel 17-18: David's inside agent buys time for him to prepare. Absalom's forces are defeated.

Ahithophel advises to attack David immediately, offering to lead the attack himself, seeking to kill David directly.
Hushai (David's agent) advises differently, reminding Absalom that David and his men are skilled warriors. Instead, Absalom should wait, recruit more men from around the country, and then attack with Absalom leading (an appeal to his pride), using overwhelming numbers, and destroy all of David and those that followed him.
This, of course, was a delaying tactic, but it was designed by God to allow David to flee further away, regroup, and receive supplied from foreign support.
Later, during the ensuring battle, Absalom's forces were dealt a bitter defeat as they fled into the forests -- the worst place to try and engage in battle.
Absalom is seen fleeing, and when this is reported to Joab, Joab runs three daggers through his heart, and his men beat him up viciously, so that if the body was discovered, it would look like someone fallen in battle. They bury him in a pit covered with stones.
Although David and his forces are victorious, David is thrown into despair and remorse at the news of his son's death. His guilt and agony of not totally forgiving his son earlier now haunts him as he now will never have a chance of reconciliation.