The Star Wars prequel trilogy is fundamentally a tragedy, chronicling the final years of an ailing Republic usurped by fascism. A major part of this tragedy is the destruction of the Jedi Order, guardians of peace and justice throughout the galaxy, and their displacement by the forces of the Empire.
However, the story isn't a hagiography of the Jedi either, their depiction is that of a noble but ultimately misguided group, who are so blinded by their own hubris they fail to prevent their own destruction. What are the most damning ways the Jedi failed to prevent Darth Sidious's rise? Let's take a look and find out.
10 Building The Jedi Temple Atop A Sith Shrine
In an unmade story arc of The Clone Wars starring Ahsoka Tano, it would be revealed that the Jedi Temple was built atop an ancient Sith Shrine, constructed during the days when Coruscant was ruled by the Sith. The Jedi hoped that constructing a beacon of light over a concentration of darkness would dilute the Sith Temple's influence, but it instead allowed the darkness to fester undisturbed. It was the dark power emanating from the Shrine which Palpatine called upon to shield his presence from the Jedi and cloud their judgement, preventing them from discovering him until it was too late. While The Clone Wars episodes remain unfinished, the presence of the Sith Shrine was canonized by James Luceno in his 2014 novel Tarkin.
9 Dogmatism
Despite their ultimately noble intentions, the Jedi throughout prequel-era stories are depicted as whole to be a dogmatic, monastic order. While some exceptions with the Order exist, as a whole they hew to an extreme version of their teachings, one which forbids a Jedi to have attachments and calls for emotions of any sort to be suppressed. The training was so absolute that new Jedi recruits were taken as infants, to ensure they'd have no bonds to the outside world by the time their training was underway. While such emotional bonds were viewed as a path to the Dark Side, it ultimately left them detached and indifferent to the denizens of the very galaxy they were trying to save, and unable to help one of their own, Anakin Skywalker, when he most needed it.
8 Believing The Sith Extinct
Between the Galactic Republic's founding and the few decades before its fall, the Jedi believed their mortal enemies, the Dark Side, to be extinct. When Qui-Gon Jinn came before the Jedi Council claiming to have faced a Sith on Tatooine, the assertion was dismissed out of hand, for the Jedi claimed the Sith couldn't exist without their knowledge.
How wrong they were, for the Sith had never ceased, but simply slinked into the shadows, hiding and biding their time until they could exact their revenge on the Jedi Order. By the time the Sith revealed themselves again, their plans were too far in motion for the Jedi to stop them.
7 Alienating Dooku
Count Dooku may have become of the greatest enemies of the Galactic Republic and the Jedi Order in their final years of existence, but he was once a Jedi himself, and not just any Jedi, but one of the most respected masters in the entire Order. However, he grew frustrated with the corruption and impotence of the Republic Senate, and especially his own order's complacent refusal to do anything to abate it. It was thus that Dooku fell under the sway of Darth Sidious, leaving the Jedi for the Sith, becoming Darth Tyranus, and playing a key role in the events that would bring about the downfall of the Jedi Order.
6 Not Investigating The Clones' Origin
When investigating the attempted assassination of Senator Padmé Amidala, Obi-Wan Kenobi follows a trail that leads him to planet Kamino. There he finds a massive Clone Army commissioned for the Republic in the name of the Jedi, despite the Jedi Council never authorizing the creation of such an army. The only clues the Jedi have is that deceased Jedi Master Sifo-Dyas ordered the Clones shortly before his death, while another man named Tyranus recruited bounty hunter Jango Fett to be the army's template.
The Jedi don't bother to investigate the army's origin further for another three years until, by chance, Sifo-Dyas' Lightsaber is discovered on a moon light-years away from where he supposedly died. While the subsequent investigation into that leads them to discovering "Tyranus" to in fact be Count Dooku, by then Jedi admit it is too late for them to do anything but continue playing the Sith's game to fruition.
5 Not Trusting Anakin
The Jedi Council had a frail relationship with Anakin Skywalker from the beginning; brought before them at age nine, long after the Jedi were comfortable inducting a recruit into the Order, the Council immediately sensed much fear in the supposed Chosen One. While Obi-Wan was eventually permitted to train Anakin, the Council, fearful of what Anakin's future held, only went out of their way to push into the welcoming arms of the Dark Side. From keeping him in the dark when Obi-Wan faked his death to go undercover to arbitrarily denying him the rank of Jedi Master, the council has only themselves to blame for Anakin's fall.
4 Turning Their Back On Ahsoka
More than any of the aforementioned choices, it was this that destroyed the relationship between Anakin and the Jedi Council beyond reparation. After Skywalker's apprentice, Ahsoka Tano, was framed for treason and murder, the Jedi Council voted to expel her from the Order and hand her over to the Senate for trial and execution. Anakin conducted an independent investigation and proved his Padawan's innocence, but the trauma of the experience coupled with the Council's paltry non-apologies was too much for Ahsoka, and she declined re-admittance into the Order. This not only shattered Anakin's trust in the council to make the right choices, but deprived him of one of his closest friends when he needed her the most.
3 Not Suspecting Palpatine
The Jedi are aware that there is a Sith Lord by the name of Darth Sidious, Master to Maul and Dooku alike, who is pulling the strings of the Clone Wars. Clearly such a figure must have a wide range of influence if he is able to kickstart a galaxy-spanning war and bring conglomerates like the Trade Federation to their heel. Yet, quite literally the most powerful man in the galaxy apparently never entered their suspect list.
Even as the Jedi grow concerned with the Chancellor centralizing more and more power within his office, to the point they were considering forcibly removing him from office, the thought of him being Sidious never crosses their mind, considering Mace Windu is left flabbergasted when Anakin reveals the truth to him.
2 Rushing To Kill Sidious
After Anakin reveals the truth to Mace Windu about Palpatine, the Jedi Master hastily scrambles to the Chancellor's office, ostensibly to arrest him but with assassination a clear Plan B if taking him alive fails to be an option. While the merits of killing the Dark Lord are a discussion best left to another time, Windu and the three Jedi accompanying him apparently didn't bother to inform any other Jedi or friendly Senators of the truth before they charged in, Lightsabers blazing. This allows Palpatine to frame the arrest as a Coup d'état, execute Order 66, and finalize the transition of the Republic into the Empire.
1 Allowing Themselves To Become Soldiers
As Mace Windu puts it, the Jedi are "keepers of the peace, not soldiers." Yet, once the Clone Wars commence, the Jedi permit themselves to become exactly that, being granted ranks with the Grand Army of The Republic and battling the Separatists alongside the Clones. This is clear moral failing on the Jedi's part, for they allowed themselves to become tools of war, subsumed by violence, instead of the peace preservers they are intended to be. As Luke Skywalker demonstrates in both his confrontations with Palpatine and Kylo Ren, a true Jedi would discover a way to defuse the situation without resorting to violence.