Scripture is hard to interpret. I am loathe to admit that God is bipolar (so to speak), because God does not contradict Himself. I would answer this in saying God has greater wisdom than us and knows what is best for all people--the Assyrians made war against the Israelites and the Babylonians enslaved the nobles. God delivered the Israelites into their hands because of their iniquities. Couldn't the other nations be delivered into the Israelites' hands because of their iniquities? This punishment could be enough to cause the people to repent, or cause others to repent.
I don't know if my answer is adequate enough to address all the questions you have with the interpretation of Scripture, but if I think of something, I'll post it.
