Astrology is, generally speaking, a way to ascertain knowledge about affairs and events that happen here on earth. It's like trying to figure out how a ball would behave if let go - for if we knew how the ball would behave then we have a predictive power for the future. The hope, ultimately, is to use such a tool in order to have better outcomes than would happen without. The problem, though, is in the notion of a causal mechanism. There's nothing linking the stars to personalities or events, except through the practice of astrology itself. The belief that Mars ascending might signal a good time to go to war will mean that looking back there will be a correlation between war and the ascension of Mars. It's causation, in other words, is entirely imposed by the pattern itself.
It's not to say that one needs to know a mechanism in order for the pattern to be meaningful, after all washing hands helped reduce infection before the germ theory of disease was known. But the lack of mechanism, combined with the tautological nature of causation based on the explanation is what makes astrology an unwarranted belief.