By the end of Star Wars: Revenge of the Sith, Palpatine had accomplished his life’s purpose. He had destroyed the Jedi, dismantled the Republic and named himself the head of a new Galactic Empire. In the process, he had corrupted Anakin Skywalker – the Jedi’s Chosen One – to his side and pronounced him Darth Vader. Not everything, however, worked according to Palpatine’s design.
During a catastrophic duel with Obi-Wan on Mustafar, Vader was nearly killed and left for dead. Nevertheless, Palatine improvised and made the best of a bad situation by turning Vader into a monster. Towering over any normal human in his all-black, cybernetic suit, Vader was a faceless menace behind an expressionless mask. Even still, none of that prevented numerous Imperials from questioning Vader’s authority over the years, which eventually necessitated Vader being given an official rank so that he could sit atop the Imperial military hierarchy.
It started early on when a few Imperial officers disliked how Vader had appeared out of nowhere and gained command with unnatural speed, especially since Vader was technically outside of the military’s structure. The story was told in Star Wars: Darth Vader (2017) #12 how Imperial officers had executed multiple, failed assassination attempts against the newly dubbed Sith Lord. So, Palpatine called an assembly of all his elite officers, introduced Vader and said “A command from [Vader] is as a command from me.”
Then, Vader took matters into his own, mechanical hands. He called five randomly selected officers to the front of the assembly and killed them simultaneously with a Force choke. He didn’t know which officers were responsible for the assassination attempts again him, so he made an example of what he would do to anyone that opposed him. Those five officers were simply collateral damage of his need to make a statement.
Clearly, that wasn’t the only time that someone opposed Vader because even during A New Hope, Admiral Motti questioned Vader’s ability to locate the rebel base and insulted Vader’s use of the Force. Vader quickly shut him up with a Force-choke as well and only relented when Tarkin asked him to stop.
All of this necessitated something more. Palpatine established his Empire on order, and Vader couldn’t carry out the Emperor’s directives if he was constantly being questioned. So, shortly after the Battle of Yavin, Palpatine gave him the title of Supreme Commander of the Imperial Forces. Technically, the Emperor held absolute authority as Head of State, Head of Government and the Commander in Chief. However, Vader’s new title essentially declared him to be the second most powerful figure in the Empire. That title truly brought all questions to an end. Vader wasn’t part of the military hierarchy, but his rank still gave him absolute control over every piece of the Imperial Military.