We learn that there have been two Sith lords secretly since the defeat of the Sith by the Jedi generations before TPM. In this time the Jedi have maintained peace and order through their use of the force. Suddenly, in TPM things are getting clouded in the force which the Jedi claim to be out of balance. They say Anakin is the chosen one who will restore balance to the force. But perhaps the prophecy is missread--everything was fine until this period--what has changed? Anakins birth! Actually it is the coming of the "chosen one" that throws everything out of wack. Palpatine already existed and things were still ok, when TPM starts, Anakin is already 9, so things have been shifting already for 9 years. Balance is restored when Palpatine is destroyed, but at virtually the same time, Anakin/Vader dies. So perhaps it is really Anakin's presence that causes the imbalance in the force and Qui-Gon and the Jedi should have realized this and not trained Anakin, but rather hid him from the Sith or better yet, though not the Jedi way, killed him and there never would have been an empire!