This implies a united invasion by the whole Philistine force. Hence, the terror of the Israelites. (Compare Judges 15:11.)
Related Commentaries of 1 Samuel 7:7
1 Samuel 7:7
7 And when the Philistines heard that the children of Israel were gathered together to Mizpeh, the lords of the Philistines went up against Israel. And when the children of Israel heard it, they were afraid of the Philistines.