Boa pergunta.
Você pergunta:
How did just one (or 2 if you count trainees) Sith Lord - even as powerful as Sidious - diminish all of Jedi's ability to use the Force, seemingly everywhere in the Galaxy?
A resposta é que ele não pode. :)
Na verdade, ele não mudou literalmente as habilidades de força dos Jedis. Ele mudou a própria Força. Veja esta passagem em A vingança de Sith :
And while Palpatine answered, Mace Windu reached into the Force. To Mace's Force perception, the world crystallized around them, becoming a gem of reality shot through with flaws and fault lines of possibility. This was Mace's particular gift: to see how people and situations fit together in the Force, to find the shear planes that can cause them to break in useful ways, and to intuit what sort of strike would best make the cut. Though he could not consistently determine the significance of the structures he perceived—the darkening cloud upon the Force that had risen with the rebirth of the Sith made that harder and harder with each passing day—the presence of shatterpoints was always clear.
De Labirinto do mal
Yoda surrendered himself to the current of the Force. Sometimes, when the current was swift and steadfast, he could see through the eyes of his fellow Jedi, almost as if they were the Temple's remote sensors. And sometimes when the current was especially forceful, when it surged as if descending from great heights, he could hear the voice of Qui-Gon Jinn, as clearly as if he were still alive.
Master Yoda, he might say, we still have much to learn. The Force remains a code only partially deciphered. But another key has been found. We will become stronger than we have ever been...
Today was not one of those days. Today the current was interrupted by eddies and whirlpools, hydraulic traps whose roar overpowered the voices Yoda sought to hear. Today the current was not pellucid, but muddied by red soil eroded from distant shores, treacherous with obstacles, tainted. Though he was scarcely aware of it, his eyelids were squeezed tight, his eyeballs dancing beneath as if incapable of focusing on any one thing.
He had an image of himself drawing aside a veil only to find another, and another beyond that. The dark side frustrated his every effort to see clearly. The experience was still something new to him. Even though he'd had centuries to grow accustomed to foreboding, he had lived far longer without it. The dark side never completely disappeared - - it scratched at the surface like an insect crawling across a transparisteel panel - - and he had been able to sense its incremental increases in strength when the Jedi erred, or when the Republic erred, and soon the two were hand in hand.
Os Jedi são os servos do lado da luz, a força. Quanto mais fraco fica, mais difícil é ver claramente, já que o lado negro faz tudo para enganá-los.
O Sith fez o lado negro mais strong.
Eles usaram três métodos para fazer isso:
Forçar técnicas:
A Força é influenciada por todos os seres vivos, é por isso que todos os lugares não caem da mesma forma na Força. Se um lugar é habitado por seres cheios de medo, raiva e ódio, o lugar se tornará strong no lado escuro (Korriban). Para um ser strong na Força, uma única vez pode ser suficiente: o lugar onde a mãe de Anakin morreu permaneceu contaminado por pelo menos décadas. A partir do estudo da Força, é possível fazê-lo conscientemente, tanto Jedi (Shaak Ti em Felucia) quanto Sith fizeram isso.
Plagueis e Sidious fizeram o mesmo para toda a galáxia ( Darth Plagueis ):
The question of whether he and Sidious had discovered something new or rediscovered something ancient was beside the point. All that mattered was that, almost a decade earlier, they had succeeded in willing the Force to shift and tip irrevocably to the dark side. Not a mere paradigm shift, but a tangible alteration that could be felt by anyone strong in the Force, and whether or not trained in the Sith or Jedi arts. The shift had been the outcome of months of intense meditation, during which Plagueis and Sidious had sought to challenge the Force for sovereignty and suffuse the galaxy with the power of the dark side. Brazen and shameless, and at their own mortal peril, they had waged etheric war, anticipating that their own midi-chlorians, the Force’s proxy army, might marshal to boil their blood or stop the beating of their hearts. Risen out of themselves, discorporate and as a single entity, they had brought the power of their will to bear, asserting their sovereignty over the Force. No counterforce had risen against them. In what amounted to a state of rapture they knew that the Force had yielded, as if some deity had been tipped from its throne. On the fulcrum they had fashioned, the light side had dipped and the dark side had ascended.
A disseminação do caos
Então nós vemos, seres podem corromper a Força. No primeiro caso, alguns seres imensamente poderosos podem fazer uma mudança galáctica. Por outro lado, indivíduos fracos, mas extremamente numerosos, podem fazer o mesmo. Então os Sith tiveram que espalhar a miséria para criar medo, raiva e ódio.
“Through us, the powers of chaos are harnessed and exploited. Dark times don’t simply emerge, Sidious. Enlightened beings, guiding intelligences manipulate events to bring about a storm that will deliver power into the hands of an elite group willing to make the hard choices the Republic fears to make. Beings may elect their leaders, but the Force has elected us.” He glanced at his apprentice. “Remember, though, that a cunning politician is capable of wreaking more havoc than two Sith Lords armed with vibroblades, lightsabers, or force pikes. That is what you must become, with me advising you from the dark.”
Desde a refondação da ordem de Bane, os Sith têm fomentado revoltas, encorajando a escravidão, tornando as instituições políticas disfuncionais, aumentando as tensões sociais, e assim por diante, culminando com as Guerras Clônicas. Isso deu força ao lado negro.
A Queda dos Jedi
Da vingança dos Sith:
Mace looked even grimmer than usual. "Since the fall of Darth Bane more than a millennium ago, there have been hundreds of thousands of Jedi-hundreds of thousands of Jedi feeding the light with each work of their hands, with each breath, with every beat of their hearts, bringing justice, building civil society, radiating peace, acting out of selfless love for all living things-and in all these thousand years, there have been only two Sith at any time. Only two. Jedi create light, but the Sith do not create darkness. They merely use the darkness that is always there. That has always been there. Greed and jealousy, aggression and lust and fear-these are all natural to sentient beings. The legacy of the jungle. Our inheritance from the dark."
Esta citação é preenchida com propaganda Jedi IMHO, mas ainda há um ponto válido. Os Jedi tornaram o lado da luz muito mais strong apenas por existirem, e fizeram esforços ativos para torná-lo ainda mais strong. As Guerras Clônicas foram a solução. Muitos grandes Jedi caíram por causa da guerra (Dooku, Deepa, Vos), e até mesmo aqueles que não se desviaram onde estavam contaminados:
De Labirinto do Mal
Yes, just as Yoda had sensed after Qui-Gon Jinn and Obi-Wan had freed him from slavery on Tatooine and brought him to the Jedi Temple, he had a lot of anger in him. But what Yoda failed to realize was that anger could be a kind of fuel. In peaceful times Anakin might have been able to bridle his rage, but now he relied on it to drive him forward, to transform him into the person he needed to be. Cut off the head. Twice he might have been able to kill Dooku himself had Obi-Wan not held him back. But he didn't hold that against his former Master.