Last King of Iran?

I just finished reading a very interesting book called "All the Shah's Men", about the last king of Iran Reza Pahlavi. I was just wondering with Mossadegh in power and democracy around the corner why did america and britain stage a coup d'etat and remove mossadegh and reinstate the shah. Although th Shah helped modernize Iran and return some of the Glory of the Persian Empire but why did they ruin a chance for democracy almost 60 years ago in Iran. IF it wasnt for them Islamic revolution wouldnt have happend and we would have had a much better and more influential ally in the Middle east seing that Iranians and Israelis are the only non-arabs in that rewgion?