The "strengthening" Eglon was the special work of God, and because Israel "had done evil," etc. Samuel's comment on the event is to the same effect 1 Samuel 12:9.

The children of Israel did evil - They forgat the Lord and became idolaters, and God made those very people, whom they had imitated in their idolatrous worship, the means of their chastisement.

The Lord strengthened Eglon the king of Moab - The success he had against the Israelites was by the especial appointment and energy of God.

He not only abandoned the Israelites, but strengthened the Moabites against them. Eglon is supposed to have been the immediate successor of Balak. Some great men have borne names which, when reduced to their grammatical meaning, appear very ridiculous: the word עגלון Eglon signifies a little calf!

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.

In vv. 12-30 the subjugation of the Israelites by Eglon, the king of the Moabites, and their deliverance from this bondage, are circumstantially described. First of all, in Judges 3:12-14, the subjugation. When the Israelites forsook the Lord again (in the place of וגו את־הרע ... ויּעשׂוּ, Judges 3:7, we have here the appropriate expression ... הרע הרע לעשׂות, they added to do, i.e., did again, evil, etc., as in Judges 4:1; Judges 10:6; Judges 13:1), the Lord made Eglon the king of the Moabites strong over Israel. על חזּק, to give a person strength to overcome or oppress another. כּי על, as in Deuteronomy 31:17, instead of the more usual אשׁר על (cf. Jeremiah 4:28; Malachi 2:14; Psalm 139:14). Eglon allied himself with the Ammonites and Amalekites, those arch-foes of Israel, invaded the land, took the palm-city, i.e., Jericho (see at Judges 1:16), and made the Israelites tributary for eighteen years. Sixty years had passed since Jericho had been burnt by Joshua. During that time the Israelites had rebuilt the ruined city, but they had not fortified it, on account of the curse pronounced by Joshua upon any one who should restore it as a fortress; so that the Moabites could easily conquer it, and using it as a base, reduce the Israelites to servitude.

And the children of Israel did evil again in the sight of the LORD: and the LORD {g} strengthened Eglon the king of Moab against Israel, because they had done evil in the sight of the LORD.

(g) So that the enemies of God's people have no power over them, but by God's appointment.

3:12 Strengthened Eglon - By giving him courage, and power, and success against them.

Jud 3:12-30. Ehud Slays Eglon.

12-14. the children of Israel did evil again in the sight of the Lord-The Israelites, deprived of the moral and political influences of Othniel, were not long in following their native bias to idolatry.

the Lord strengthened Eglon the king of Moab-The reigning monarch's ambition was to recover that extensive portion of his ancient territory possessed by the Israelites. In conjunction with his neighbors, the Ammonites and the Amalekites, sworn enemies of Israel, he first subjected the eastern tribes; then crossing the Jordan, he made a sudden incursion on western Canaan, and in virtue of his conquests, erected fortifications in the territory adjoining Jericho [Josephus], to secure the frontier, and fixed his residence there. This oppressor was permitted, in the providence of God, to triumph for eighteen years.

3:12-30 When Israel sins again, God raises up a new oppressor. The Israelites did ill, and the Moabites did worse; yet because God punishes the sins of his own people in this world, Israel is weakened, and Moab strengthened against them. If lesser troubles do not do the work, God will send greater. When Israel prays again, God raises up Ehud. As a judge, or minister of Divine justice, Ehud put to death Eglon, the king of Moab, and thus executed the judgments of God upon him as an enemy to God and Israel. But the law of being subject to principalities and powers in all things lawful, is the rule of our conduct. No such commissions are now given; to pretend to them is to blaspheme God. Notice Ehud's address to Eglon. What message from God but a message of vengeance can a proud rebel expect? Such a message is contained in the word of God; his ministers are boldly to declare it, without fearing the frown, or respecting the persons of sinners. But, blessed be God, they have to deliver a message of mercy and of free salvation; the message of vengeance belongs only to those who neglect the offers of grace. The consequence of this victory was, that the land had rest eighty years. It was a great while for the land to rest; yet what is that to the saints' everlasting rest in the heavenly Canaan.