What Excuse Did Vader Give For His Defeat By Obi-Wan Kenobi?

Inexperience, as he tells it. Vader never concedes on screen that his own arrogance beat him on Mustafar. When he faces Obi-Wan again on the Death Star he sums up their last meeting as the learner against the master, and the grievance behind that line is one Anakin had nursed for years: that Obi-Wan and the Jedi Council were holding him back. (A New Hope; Attack of the Clones; Revenge of the Sith)

The duel that capped his ceiling#

Anakin Skywalker's defeat at Obi-Wan Kenobi's hands was one of the hinge moments of the whole galaxy. Not because it removed him, but because of what it left behind. The injuries he took on Mustafar meant that what walked away was permanently dependent on a suit, and that suit put a hard ceiling on how much of his raw potential he could ever draw on. The Chosen One the Jedi had been arguing about for a decade never got to exist. Darth Vader did instead. (Revenge of the Sith)

That is worth sitting with, because on paper Anakin should have won. By the end of the war he was widely seen as more powerful than Obi-Wan Kenobi, the Chosen One whose raw power no one in the Order disputed. There was no straightforward reading in which the student loses that fight. (Revenge of the Sith)

Why Obi-Wan actually won#

What decided it was not strength, it was temperament. (Revenge of the Sith)

Kenobi stuck to the Jedi way and kept a level head. That approach was baked into how he fought. His signature was Form III, Soresu, the defensive form built on tight controlled movements and a strict economy of action, where even a slight extra motion opens a gap for a blade or a blaster bolt to slip through. Soresu practitioners train to block, parry, absorb multiple opponents, and above all hold a calm focus that lets them answer whatever comes. It is a form designed to let an outmatched fighter survive long enough for the other one to make a mistake. (Star Wars: Absolutely Everything You Need to Know; Knights of Fate)

Anakin obliged. He gave way to arrogance and overconfidence, fought angry, and made the small errors that Soresu exists to punish. Then he made the largest one. (Revenge of the Sith)

Vader's own account#

Here is the interesting part. Vader lived with that defeat for decades, and nothing he says on screen puts it down to his own arrogance. (Revenge of the Sith; A New Hope)

His framing is experience. Aboard the Death Star he tells Obi-Wan that when they parted he was only the learner, and now he is the master. No source has him lay out a fuller excuse than that, but the line fits a complaint Anakin had been making since long before Mustafar: that Obi-Wan was holding him back, and that the Jedi Order would not give him the standing he felt he had earned. On that reading he did not lose to a better fighter or to his own temper. He lost because he had not finished growing, and the people he blamed for that were the ones who kept restraining him. (A New Hope; Attack of the Clones; Revenge of the Sith)

It is a self-serving answer, and it is also not a completely invented grievance. Anakin genuinely had been held back, and the friction was real. He was placed on the Jedi Council after Count Dooku's death and then denied the rank of Master, an insult that ate at him and pushed him further towards Palpatine. Kenobi meanwhile had spent the war as a Jedi General and a Council member, running campaigns, and had built a personal rivalry with General Grievous that put him across from one of the deadliest duellists of the era over and over again. (Revenge of the Sith)

So the learner line takes a true fact, that the Order restricted him and gave Kenobi more room, and stretches it into a full explanation for the worst day of his life. (A New Hope; Revenge of the Sith)

What the excuse tells you about him#

The excuse is more revealing than the truth would have been. A man who could admit he lost his head, and that it cost him everything, has some route back. A man who blames the Council for never letting him train hard enough has only one direction to travel, and it runs towards the Sith. (Revenge of the Sith)

It also explains a lot about how Vader operated afterwards. If the lesson you take from Mustafar is that you were denied opportunities rather than that your rage betrayed you, you do not fix the rage. You go looking for opportunities. The Great Jedi Purge and everything that followed reads slightly differently once you know he thought of himself as a fighter who had been unfairly benched. (Revenge of the Sith; Obi-Wan Kenobi, "Part III")

Obi-Wan, for his part, drew a different conclusion from the same duel. He carried it as a failure rather than a win, went into exile on Tatooine, and spent the next two decades guarding the boy who would eventually finish what he could not. (Revenge of the Sith; A New Hope)

The rematch that should have settled it#

If the excuse were true, the fix would be obvious. Get more experience, come back, win. (A New Hope)

Vader did exactly that, and in Canon he got more than one rematch. Ten years after Mustafar he caught Kenobi on Mapuzo, beat him and set him on fire. Soon afterwards, on a barren moon, Kenobi beat him again and left him broken in his helmet. Nine years after that, aboard the Death Star, Vader had every advantage the original duel denied him. He was vastly more practised and facing an opponent well past his prime, and he cut Kenobi down without much difficulty. (Obi-Wan Kenobi, "Part III"; "Part VI"; A New Hope)

It settled nothing. Kenobi's body vanished at the moment of the strike, and Vader was left standing on an empty robe with a feeling he had no framework for. We have covered what that did to him separately, but it belongs here too, because it closes the loop on the excuse. (A New Hope)

He got the experience he claimed to be missing. He won the fight he claimed the Order had cost him. And the man he beat was still, by any measure that mattered, ahead of him. The problem was never the training record. It was the assumption that a duel is a thing you win by being better at duelling. (A New Hope)