Summary
Palpatine’s love of superweapons and duplication of the Death Star meant he made a massive mistake inStar Wars. There’s a sense in which the Death Star was Palpatine’s folly; the creation of the first Death Star consumed enormous amounts of time and resources that could have been better spent in countless other ways. It didn’t help that he never expected the Death Star to be so swiftly destroyed, and soPalpatine made the mistake of disbanding the Senate. He believed he no longer needed to keep his opponents occupied with politics in the face of a superweapon.
Sith seem to be obsessed with superweapons. Consequently, the Emperor failed to learn from his mistake; after the destruction of one Death Star, he simply had another one made. But he made another crucial mistake with the second Death Star, a repeat of the first that doomed the Empire. It’s recounted in Chris Kempshall’s excellent bookThe Rise & Fall of the Galactic Empire- and it shows how foolish Palpatine really was.

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Palpatine Took Too Many Of His Top Advisors To Endor With Him
Despite what happened to the first Death Star, Palpatine still managed to overlook the dangers of stationing way too many of his important leaders together on the second one. After it was destroyed by the Rebel fleet and the Emperor was gone, the Empire had no leaders to speak of. “Palpatine had taken many of his closest advisors - like Sate Pestage and Sim Aloo - with him to Endor, where they’d also perished,” Kempshall points out.Without a real leader to assume control, the Empire’s days were numbered.
Perhaps this move was fueled by Palpatine’s pride and the idea that his presence alone would be enough to prevent another catastrophe. Palpatine put himself in the same danger that the rest of his officials would be in, implying that he truly didn’t believe that the Rebels could destroy another Death Star without Luke Skywalker. But by shifting all his attention to Vader’s attempts to turn his son to the dark side,the unprotected Death Star II became a time bomb just waiting to destroy the Empire from the inside out.

The Death Of So Many Key Leaders Destroyed The Imperial Command Structure
Not only does the Empire lose its superweapon when the Rebels take out the first Death Star, but many Empirical leaders are killed in the process.The death of Grand Moff Tarkinand other important officers on the first Death Star gave the Rebels the edge they needed over the Empire, allowing them to thrive and become more organized and advanced. The Death Star II makes the exact same mistake, this time at the cost of the Empire in its entirety.
The Empire was seriously disadvantaged after the loss of the Emperor, Darth Vader, and almost every other high-ranking officer. It took only a year for the Empire to collapse, destroyed at theBattle of Jakku. Even the few politicians that survived the second Death Star weren’t enough to keep the Empire from crumbling, according to Kempshall.
“All that was now left on Coruscant, aside from the functioning bureaucracy of the Imperial state, was Mas Amedda.”
With no support or guidance, Mas Amedda quickly surrendered to the Rebels, officially absorbing the Empire into the New Republic.
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Emperor Palpatine / Darth Sidious
The senator for Naboo, Palpatine rose to power and influence during the dying days of the Republic. In reality, his public persona was just a mask; he was really Darth Sidious, greatest of the Sith Lords, and he used his political skills and Machiavellian cunning to bring down both the Republic and the Jedi. Palpatine ruled his Galactic Empire for decades, until he was betrayed by his apprentice, Darth Vader. Even this wasn’t enough to stop the Emperor, however, as he was resurrected by his followers - only to be defeated once again.