John Gill Commentary Judges 3:12

"And the children of Israel again did that which was evil in the sight of Jehovah: and Jehovah strengthened Eglon the king of Moab against Israel, because they had done that which was evil in the sight of Jehovah." — Judges 3:12 (ASV)

And the children of Israel did evil again in the sight of the
Lord
Fell into idolatry again, which was a great evil in the sight of God, and what they were prone to fall into:

and the Lord strengthened Eglon the king of Moab against Israel ;
put it into his heart to invade them, and encouraged him to it, and gave him success; what kings reigned over Moab between Balak and this king we know not: it is a commonly received notion of the Jews, that Ruth was the daughter of Eglon; see (Ruth 1:4) ; and it was about this time that Elimelech with his two sons went into Moab, and when many of those things recorded in the book of Ruth were transacted:

because they had done evil in the sight of the Lord ;
which had greatly provoked him to anger, and was the cause of stirring up the king of Moab against them.