They did not destroy the nations It appears to me that those persons are mistaken who think that the prophet is here simply giving a relation of the punishment which was inflicted upon the Jews, as if he were imputing to them the entire blame of not exterminating the nations, in consequence of their not deserving the honor of obtaining any more victories over them. But he rather prefers another charge against them, that they had been remiss in driving out the heathen, or more probably that they had not obeyed the Divine command to root them out of the land. Now that the cup of the iniquity of the Amorites was full, it was the purpose of God that they should be exterminated, lest their society might prove injurious to the holy people. For God, having chosen that land for a habitation to himself, intended that it should be holy and purified from all defilement. In refusing, therefore, to execute the vengeance enjoined upon them, the people showed their willingness to associate with the uncircumcised inhabitants of Canaan. In manifesting such indifference about God's command respecting the driving out these nations, they gave just cause for his anger waxing hot against them. Behold, saith he, I have commanded all these nations to be cut off by the sword; and now, because ye have not obeyed my voice, "they shall be pricks in your eyes, and thorns in your sides, and shall vex you in the land wherein ye dwell," Numbers 33:55 The not destroying all these nations, but permitting some of them to remain, might appear to be an act of mercy; but in thus acting, the people were guilty of neglecting to execute God's righteous vengeance upon them, and of leaving the land liable to be polluted with their abominations. From these things it ought to be noticed, that there are two extremes in which men are apt to indulge, either in being unnecessarily over rigorous, or in defeating the ends of justice by too great lenity. We must, therefore, adhere strictly to God's command, if we would desire to shun both extremes. For if the Israelites are condemned for sparing some of these nations wholly, what are we to think of those judges who, from a timid and apathetic attention to the responsible duties of their office, exercise too much lenity to a few persons, thus weakening the restraints of the inlets to vice, to the great detriment of the public weal?

They did not destroy the nations - The Canaanites, Hivites, Jebusites, etc.; the nations that inhabited the land of Canaan.
Concerning whom the Lord commanded them - The command on this subject was positive; and it was to destroy them, to spare none of them. Numbers 33:52; Deuteronomy 7:5, Deuteronomy 7:16.

They did not destroy the nations,.... Here begins an account of their sins and provocations, after they were settled in the land of Canaan. They did not destroy the inhabitants of the land, of the seven nations; whose land was given to them as an inheritance, and of which the Canaanites were dispossessed for their sins, and to be destroyed.
Concerning whom the Lord commanded them; that they should destroy them; the command is in Deuteronomy 7:1. God's commands are to be obeyed; they are neither to be added to, nor diminished from; his commands are transgressed and violated by sins of omission or commission; the Israelites might plead mercy, but this was no excuse to an express command: the same sin Saul was afterwards guilty of, with respect to one of these nations, 1-Samuel 15:2. Those spiritual Canaanites, the sinful deeds of the body, are to be mortified, and not indulged and spared, Colossians 3:5.

The conduct of the Israelites in Canaan, and God's dealings with them, show that the way of sin is down-hill; omissions make way for commissions: when they neglected to destroy the heathen, they learned their works. One sin led to many more, and brought the judgments of God on them. Their sin was, in part, their own punishment. Sinners often see themselves ruined by those who led them into evil. Satan, who is a tempter, will be a tormentor. At length, God showed pity to his people for his covenant's sake. The unchangeableness of God's merciful nature and love to his people, makes him change the course of justice into mercy; and no other change is meant by God's repentance. Our case is awful when the outward church is considered. When nations professing Christianity, are so guilty as we are, no wonder if the Lord brings them low for their sins. Unless there is general and deep repentance, there can be no prospect but of increasing calamities. The psalm concludes with prayer for completing the deliverance of God's people, and praise for the beginning and progress of it. May all the people of the earth, ere long, add their Amen.

They not only failed to expel the heathen, as God
commanded-- (Exodus 23:32-33), literally, "said (they should)," but conformed to their idolatries [Psalm 106:36], and thus became spiritual adulterers (Psalm 73:27).

The sins in Canaan: the failing to exterminate the idolatrous peoples and sharing in their idolatry. In Psalm 106:34 the poet appeals to the command, frequently enjoined upon them from Exodus 23:32. onwards, to extirpate the inhabitants of Canaan. Since they did not execute this command (vid., Judges 1:1), that which it was intended to prevent came to pass: the heathen became to them a snare (mowqeesh), Exodus 23:33; Exodus 34:12; Deuteronomy 7:16. They intermarried with them, and fell into the Canaanitish custom in which the abominations of heathenism culminate, viz., the human sacrifice, which Jahve abhorreth (Deuteronomy 12:31), and only the demons (שׁדים, Deuteronomy 32:17) delight in. Thus then the land was defiled by blood-guiltiness (חנף, Numbers 35:33, cf. Isaiah 24:5; Isaiah 26:21), and they themselves became unclean (Ezekiel 20:43) by the whoredom of idolatry. In Psalm 106:40-43 the poet (as in Nehemiah 9:26.) sketches the alternation of apostasy, captivity, redemption, and relapse which followed upon the possession of Canaan, and more especially that which characterized the period of the judges. God's "counsel" was to make Israel free and glorious, but they leaned upon themselves, following their own intentions (בּעצתם); wherefore they perished in their sins. The poet uses מכך (to sink down, fall away) instead of the נמק (to moulder, rot) of the primary passage, Leviticus 26:39, retained in Ezekiel 24:23; Ezekiel 33:10, which is no blunder (Hitzig), but a deliberate change.