What does 2 Samuel 5:18 mean?

In the time of Saul and David, the Philistines mostly occupied the western coast. They made occasional forays into Judah and the northern tribes. David fought them consistently while he served Saul, but not so much during the years when he and his men hid from the king in the wilderness. We're not told if David fought the Philistines during the seven years he was king of Judah in Hebron.

Now, however, David is king of all Israel, including those vulnerable northern tribes, and the Philistines are concerned. They need to weaken Israel's political unity and stop David before he can grow much stronger.

The Philistine armies spread out in the Valley of Rephaim, northeast of Jerusalem. This strategic location might allow them to cut off potential reinforcements from the territory of Judah in the south. Spreading out implies setting up their camps and settling in for as long as it will take to root David out and defeat him. As always, the Philistines come well-equipped and with large numbers.

The Philistines have their strategy, but David has his God. Before David responds, he asks if God even wants him to fight. And, if so, will he win? God gives him confirmation, and the Philistines lose spectacularly (2 Samuel 5:19). David says, "The Lord has broken through my enemies before me like a breaking flood" (2 Samuel 5:20).