Emperor Palpatine – Star Wars’ most powerful Sith Lord – wasn’t just renowned for his skill with a lightsaber or his control of the Force: he was a class-A schemer. The way that he played the long game to weaken the Republic from within and destroy Jedi Oder was the culmination of a thousand years’ worth of Darth-Bane-designed subterfuge, and it was downright impressive. However, it seemed like Palpatine used up all of his plotting acumen in the prequels because by the time that the Rebel Alliance organized to oppose him, he made two critical, tactical errors that led to the Empire losing the Galactic Civil War.
For Palpatine, intimidation was always paramount. On a personal level, his Dark Side presence overwhelmed everyone around him, but Palpatine wanted to intimidate, threaten and subjugate the Star Wars galaxy in a way that had never been seen. So, he commissioned the first Death Star. In his hubris, Palpatine thought that no one could ever destroy or even oppose such a destructive, technological masterpiece but the Rebels did exactly that. The open exhaust port that led directly to its reactor core was Palpatine’s first error, but that’s mildly excusable because of Galen Erso’s design deception. However, Palpatine’s lack of planning around the Death Star II was his biggest failure as Emperor.
Likely embarrassed by the destruction of the Death Star I, Palpatine decided to go for round two. He commissioned the Death Star II as an even bigger monstrosity than its predecessor. With the traitor Galen Erso dead this time around, Palpatine was confident that nothing could stop his new, impervious battle station. So, he allowed the Death Star II’s plans to be “stolen” by the Rebels in a bit to draw them out of hiding. By, falsely communicating that the Death Star II was not yet operational, Palpatine’s plan worked to perfection and the Rebels prepared their preemptive strike.
Palpatine’s plan was working to perfection as the Rebels began their assault, and he told Luke as much aboard the Death Star II. Unfortunately, for Palpatine, though, his plan was not all-encompassing. The happenings down on Endor were far from being up to snuff and showed the ineptitude of the Imperial military.
Presumably, the Empire chose Endor as the staging ground for the completion of the Death Star II because the location was relatively secluded in the Outer Rim. For that matter, using a shield generator to protect the vulnerable battle station was a good plan, but, for some reason, the Empire put little to no effort to secure its location on the moon. The forest around the Imperial installation wasn't cleared to make attacks more difficult, and there was a minimal defensive presence on the moon. That was Palpatine’s biggest tactical error because Endor was clearly occupied and hostile territory.
According to Star Wars: Absolutely Everything You Need to Know, Endor was filled with a population of some thirty million Ewoks, and it only took two hundred of the war-like Teddy Bears from Bright Tree Village to disrupt the defense of the shield generator in Return of the Jedi. Bottom line: they were an obvious threat. So, the question should be why didn’t the Empire take them seriously beforehand?
It wasn’t even like the Ewoks’ attack at Star Wars' Battle of Endor was a one-time occurrence. They used large catapults, gliders, swinging logs and bows and arrows. They were ready for a fight and were likely engaging in guerrilla warfare long before Leia found Wicket. So, the Empire should have been ready for an attack at all times. Yet, the Empire must've ignored the threat because there was no indication that they had made any significant attempts to eradicate Bright Tree Village. Doing so would have been easy, and the fact that the Ewoks were allowed to remain a threat was clearly a huge, tactical error.
Palpatine’s failure to plan for the Ewoks’ threat was his biggest mistake as a tactician. He should have eradicated the Ewoks in Bright Star Village long before commencing his plan to draw the Rebels out of hiding. In fact, if his forces could have held the shield generator, his plan probably would have worked and the Rebels would have been destroyed. However, he didn’t take care of the Ewoks. Consequently, he lost Star Wars' Galactic Civil War and the Rebels destroyed his precious Death Star – again.