Ezra 9-10: Ezra is horrified to find that the Jews have (again) been adopting Canaanite values.

Ezra arrives to find that there is intermarriage going on, and that there are already some children.
The problem is not with ethnic purity, but many of the Canaanite spouses have not converted to worshipping Yahweh, and have brought their own idols and pagan customs into their houses.
Ezra is horrified -- since this is exactly what caused them to be conquered and deported to Babylon the last time
He enters into prayer, and eventually, enough people are concerned that a plan of action is taken.
It is a drastic plan, but the issue must be nipped before it becomes worse.
A systematic plan of investigating each intermarriage, to see if the spouses have truly left their idols, is begun.
Eventually, many Canaanite spouses are sent away -- but thankfully few children are among them.
They are returned to their families and the curse is lifted.