Given the fact that Itachi asked Obito to help him eliminate the Uchiha, why was he surprised when he knew about the secret mission? When reanimated Itachi fought against Naruto, he asked him what happened to Sasuke. Naruto proceeded to tell him about what Obito (at the time, Madara) said, and tells him the convince Sasuke to join the Akatsuki while taking revenge on Konoha for betraying his brother.

Itachi then looks surprised and says "Damn Madara, he knew about my secret mission after all".

However, given the fact that Itachi asked for his help in massacring the Uchiha, why is he so surprised? After all, he told him enough about it.
Itachi may have asked Madara's help but it was never explicitly said that he revealed the true reason behind his actions.


  Itachi may have lied to Madara like he did to Sasuke that he massacred
  all uchiha to test his powers.


Itachi may be thinking that Madara was convinced with his reasoning, while Madara knew the true reason that he was doing all this by leaf village's orders.