we know that danzo cooperate with hanzo,and hanzo knew very much information about akatsuky,so danzo was very good informed by hanzo,he knew about akatsuki but he keep the secret and dont tell to the hokage in time,to try to prevent the invasion of konoha,when poor shizune and intel divisions struggling to find any intel about pain,danzo knew very good the information,but i think that danzo dont fight because he want that hokage to be dead,so he cand take the hokage title.