דברי הימים א, פרק י״ד, פסוק ח׳

The consolidation of David's rule over the entire nation shifts the regional balance of power, sparking an immediate military reaction from the Philistines. During the seven years David ruled only in Hebron, the Philistines chose not to interfere. They assumed he was merely a local official and welcomed his rivalry with the house of Saul, hoping this internal conflict would work to their advantage [רש״י, חומת אנך]. However, the moment they learned he had been anointed king over all the Israelites, they realized he was a direct threat to their dominance, an authority they had held over the nation since the days of Samson [רש״י].

Recognizing the danger, the Philistines mobilized their entire military force. They were well aware of David's bravery in combat and understood that confronting him would require immense strength [מצודת דוד]. Their primary aim was to hunt him down, wage war, and completely remove him from his newly established kingdom [רד״ק, ביאור שטיינזלץ]. On the other hand, because David had previously been their ally and fought for the king of Gath, another perspective suggests the Philistines came to test him, demanding to know if he would continue to submit to their authority even as a ruling king [מלבי״ם].

Upon hearing of their advance, David immediately set out to meet them in battle [ביאור שטיינזלץ]. While another biblical account mentions that David went down to a stronghold, the events align perfectly. He marched out and positioned himself at a fortress near the Philistine camp [רלב״ג]. Taking a proactive approach, David actually intended to strike the Philistines first within their own territory. He moved to a stronghold in the nearby wilderness to block them from advancing further into the land of Israel. Despite his efforts, the Philistines moved faster and managed to deploy their forces in the valley just outside Jerusalem [מלבי״ם].