The names “Babylonians” and “Nebuchadnezzar” are never mentioned in the book of Lamentations, even though it was Nebuchadnezzar and his people, the Babylonians, who did all these things to Judah and Jerusalem. But Jeremiah and his fellow believers knew who really brought this disaster on them: the LORD. They also know why he did it: because of "the multitude of her transgressions." Of course, they remember the LORD’s warnings from long ago through Moses, that if his people would keep on breaking his covenant, he would give their enemies power over them (see Deuteronomy 28:13; Deuteronomy 28:15; Deuteronomy 28:44).
A terrible thing is mentioned in this verse: even children are caught and taken away! What could be a worse thing for a parent to see? But Jeremiah and his fellow believers are not speaking about these things in order to complain against God. While they are grieving, they are confessing their own sins before God.
5 Her foes have become the head; her enemies prosper, because the LORD has afflicted her for the multitude of her transgressions; her children have gone away, captives before the foe.