שמואל ב, פרק ג׳, פסוק כ״ב

A commander returning fresh from a battlefield victory often feels a surge of confidence that can lead to bold, unchecked decisions. This dynamic sets the stage for a volatile clash, as Joab arrives just after a fateful political meeting concludes in his absence. Leading David's men, Joab returns from a military raid focused on gathering spoils of war [ביאור שטיינזלץ, רש״י, מצודת דוד]. While the specific enemy is left unsaid, the battle was likely fought against the Philistines, perhaps in response to an incursion into the land of Israel, or against other neighboring nations [רד״ק, ביאור שטיינזלץ].

The massive amount of loot brought back from this campaign, combined with the timing of Abner's departure, provides the critical background for Joab's extreme actions that follow. The wealth and military triumph deeply boost Joab's morale, instilling in him the absolute confidence to act entirely on his own terms.

Furthermore, Abner's absence from Hebron is the necessary condition that allows Joab to strike. Had Abner remained under David's protection, the king would have intervened and stopped Joab from making a move. Because Abner is already on the road, Joab is handed the perfect window of opportunity and a convenient excuse to pursue him [מלבי״ם].