Star Wars has finally explained how Palpatine cut the Jedi off from the Force. A thousand years before the events of Star Wars: Episode I - The Phantom Menace, the Sith Empire fell - due just as much to in-fighting as to the Jedi. Although the galaxy believed the Sith to be extinct, in reality, they had been reformed around the Rule of Two, with Darth Bane deciding that there should only ever be two Sith at any one time - a master to wield power, and an apprentice to crave it.
The Sith were patient, and their plan took a millennium to come to fruition. Their ultimate victory is seen in the prequel trilogy, with Palpatine - in reality, the Sith Lord Darth Sidious - successfully becoming Supreme Chancellor of the Republic. He carefully manipulated events across the galaxy in order to trigger the Clone Wars, the ultimate Jedi trap. And yet, curiously, not every aspect of this trap has been fully explained. One of the most curious twists is seen at the very beginning of the Clone Wars, in the final scenes of Star Wars: Episode II - Attack of the Clones, with the Jedi Council admitting their ability to use the Force has been diminished. Most viewers assumed this was something Palpatine did, some esoteric Force power as yet unexplained - but it now seems the truth was far more remarkable.
Charles Soule's novel Light of the Jedi is the first story set in the High Republic Era, a period of time roughly 200 years before the events of Star Wars: Episode I - The Phantom Menace. One key scene sees a Jedi Master imprisoned by a fearsome enemy named Marchion Ro, who delights in taunting his captive, explaining why his ability to use the Force is diminished. "My family knew all about people like you," he tells his prisoner. "They told me what you could do, and how to resist it." Marchion Ro points to the immediate surroundings, essentially a torture chamber in which countless innocents are suffering. "They're not getting out either," he continues. "If they die, I'll just bring in more. Their job is to fill this deck with pain and anger and fear. Makes it hard for you to think, doesn't it? Hard for you to call on the Force... You don't imprison Jedi behind bars. You do it with pain."
It's a smart approach, and clearly successful, but it also has enormous implications, because it binds the Jedi into the galaxy's balance of the Force. Lucasfilm's Dave Filoni - who studied under George Lucas himself - recently explained that every living being has a part to play in the balance of the Force. "The Force naturally exists in balance," he observed. "That balance is thrown out when someone chooses to give in to their fears and then spirals out of control making selfish choice after selfish choice. Fear leads to anger, anger leads to hate, hate leads to suffering." The build-up to the Clone Wars was a time of great fear, with everyday citizens naturally disturbed at the possibility of war given the Republic was unexpectedly attempting to fund a military; thus the Jedi would have been diminished. That fear would have naturally been amplified when the Clone Wars actually began, explaining why the Jedi found themselves drastically reduced in power. And with the Jedi fighting on the front lines, surrounded by pain and fear, anger and hate, they would only have gotten weaker as the Clone Wars progressed.
This, then, is how Palpatine weakened the Jedi. It was not another Force power at all; rather, it was another aspect of the Jedi trap he had created in the Clone Wars. Darth Sidious' knowledge of the dark side essentially allowed him to outmaneuver the Jedi, and with the darkness ascendant, they found themselves becoming weaker by the day, less able to call upon the Force for wisdom, and more likely to make mistakes. Little wonder Palpatine encouraged an attack upon Coruscant shortly before initiating Order 66, aware the catastrophic loss of confidence triggered by this would have a further debilitating effect upon his enemies. Palpatine really is the most dangerous, cunning villain in all Star Wars.