the authorization to use military force in iraq was 1) punting the question to the president, who was authorized under AUMF to do many things, up to an including ground invasion - ground invasion wasn't the only option (yes, completely weak behavior by a spineless congress); 2) obtained on false pretenses about iraq's WMD capabilities.
i don't know whether bush knew about the second part, but he sure as shit knew about the first part and rushed off for the most extreme measure authorized immediately anyway.
and to answer compuwiz's question, if bush had been a strong leader maybe he'd have known about the second part instead of being led around by the nose by cheney, who, aside from being one of history's biggest assholes, was certainly a strong leader (as big assholes often are).