This verse is the start of a long list which sums up the sins of which Manasseh is guilty. It is noted that Manasseh brought back the despicable practices, which, through the Lord’s interventions, had disappeared from the land when Israel conquered the Promised Land.
Within the borders of the holy land, the same despicable practices were found as before the people of Israel arrived, but now these practices were carried out by God’s own people. That makes it worse, and contaminates the land.
2 And he did what was evil in the sight of the LORD, according to the despicable practices of the nations whom the LORD drove out before the people of Israel.