And he judged Israel in the days of the Philistines twenty years.

Wesley's Notes

15:20 He judged - That is, he pleaded their cause, and avenged them against the Philistines. Of the Philistines - That is, whilst the Philistines had the power and dominion, from which he was not fully to deliver, but only to begin to deliver them. From this place it is manifest, that in the computation of the times of the judges, the years of servitude or oppression are not to be separated from the years of the judges, but added to them, and are comprehended within them; which proposition is of great importance for clearing this difficult part of scripture - chronology.

Matthew Henry's Concise Commentary

15:18-20 So little notice did the men of Judah take of their deliverer, that he was ready to perish for want of a draught of water. Thus are the greatest slights often put upon those who do the greatest services. Samson prayed to God in this distress. Those that forget to attend God their praises, may be compelled to attend him with their prayers. Past experiences of God's power and goodness, are excellent pleas in prayer for further mercy. He pleads his being exposed to God's enemies; our best pleas are taken from God's glory. The Lord sent him seasonable relief. The place of this action was, from the jaw-bone, called Lehi. And in the place thus called, God caused a fountain suddenly and seasonably to open, close by Samson. We should be more thankful for the mercy of water, did we consider how ill we can spare it. Israel submitted to him whom they had betrayed. God was with him; henceforward they were directed by him as their judge.