Habakkuk (1:13--14) complains to God:
Your eyes are too pure to see evil, you cannot countenance oppression. So why do you countenance traitors? Why are you silent when evil people swallow up those more righteous than they? You make people like fish in the sea, like reptiles that have no ruler!
The complaint concerns the cruelty of the Babylonian troops, as they stormed through the region, capturing one nation after another.
God's reply is apparently in 2:2-20:
For the vision is meant for its appointed time; it speaks of the end, and it does not lie. It may take a while, but wait for it; it will surely come, it will not delay... ... Because you plundered many nations, all the rest of the peoples will plunder you; because of the bloodshed and violence done to the land, the city and all who live there.
So the answer is: "be patient. Some time in the future, all the nations that you've robbed will rise up and rob you".
But, this reply does not really answer the question. The question is "why do you let the Babylonians succeed in harming the other nations, and let people be like fish with no ruler?". The reply does not tell why - it just tells that, some time in the future, the "fish" will take their vengence. They will still remain like fish, only the power relations would change.
Is there another answer to Habakkuk's complaint in the book?