Spoiler from Wiki in regards to Sauron/ Saruman.
Spoiler!
The White Council long feared the power in Dol Guldur might be Sauron, but Saruman opposed assaulting it, because he knew that the Necromancer was Sauron, and he wanted to wait for him to grow in power until the One Ring revealed itself to him, so Saruman could then take it for himself. But in TA 2063 Gandalf went to Dol Guldur, and learned that the Necromancer was indeed Sauron. He then informed the White Council, and Saruman was unable to protest. The White Council attacked Dol Goldur, and Sauron, not yet powerful enough to challenge them, fled to Mordor. In TA 2460 Sauron returned there, just as the One Ring was found by Sméagol the Stoor Hobbit. However, the Ring disappeared with Sméagol under the Misty Mountains (Hithaeglir).
I find the history of Middle Earth to be so amazing. I could spend hours on the Middle Earth Wiki site.