Israel as Daughter, Bride, Virgin, or Prostitute
In parts of the Book of Jeremiah, God addresses Judah as a female, typically an adulterous wife cheating on God. Israel's even compared with a female camel or "wild ass…in her heat, sniffing the wind." (Jeremiah 2:24). (To be fair, Jeremiah also attacks men who were "like well-fed stallions, each neighing after his neighbor's wife," see 5:8). At other times, God speaks of Judah as a "virgin daughter" (14:17).
Early on, God says that Israel was once like a faithful bride:
I remember the devotion of your youth, your love as a bride, how you followed me in the wilderness, in a land not sown. Israel was holy to the Lord, the first fruits of his harvest. All who ate of it were held guilty; disaster came upon them, says the Lord. (2:3)
But this didn't last, and Israel started worshipping other gods. God says, "You have polluted the land with your whoring and wickedness" (3:2).
It's a strange metaphor in a way. Whereas the gods of other countries had wives who were also divine—goddesses like Hera, the wife of Zeus—and would occasionally stop by to impregnate individual human women, the Biblical God depicts himself as being married to his people (though he's getting a temporary divorce in Jeremiah). Also the fact that God punishes his wife for adultery and disobedience with war, famine, and disease is clearly troubling to many people. In ancient times, the devotion of a wife to her husband mirrored the devotion of a people to their ruler. This was important in holding society together, which is why adultery by the wife was so severely punished. At the same time, it's something to think about as people continue to debate the Bible and its meaning.
Israel as God's War Club
The NRSV interprets this passage as referring to Israel—but it would make more sense, in The Book of Jeremiah's case, if it referred to Babylon (since Babylon really is God's war club in Jeremiah, whereas Judah's the victim.
You are my war club, my weapon of battle: with you I smash nations; with you I destroy kingdoms; with you I smash the horse and its rider; with you I smash the chariot and the charioteer; with you I smash man and woman; with you I smash the old man and the boy; with you I smash the young man and the girl; with you I smash shepherds and their flocks; with you I smash farmers and their teams; with you I smash governors and deputies. (51:20-23)
This is pretty straightforward. Jeremiah's God is a God of War. He wants to demonstrate his superiority over other competing gods and the people who worship them. His preferred method for doing this is obvious: smashing stuff.