Was Mace Windu technically committing treason or not?

Question

In Star Wars Episode III: Revenge of the Sith, Chancellor Palpatine accuses Mace Windu and other Jedi Masters of treason for trying to arrest him. Were they actually committing treason though?

First of all, Palpatine had certainly broken the law. He assisted the Trade Federation and therefore had committed treason himself. He also took action to murder Obi-Wan Kenobi and Qui-Gon Jinn, which is surely illegal.

I don't know if Mace Windu was legally allowed to arrest Palpatine, but even if not he was trying to arrest a criminal, perhaps this could go down as a citizen's arrest.

So the question is: My lord, is that legal (to arrest Palpatine, or was it treason)?

Answer 1

The novelisation has a very slightly different version of the conversation with Windu and Palpatine. It would appear that their stated reason for arresting him was flawed, hence their arrest procedure was flawed since you can't just arrest someone for their religious beliefs in the Republic.

PALPATINE: I beg your pardon? What I am? When last I checked, I was Supreme Chancellor of the Republic you are sworn to serve. I hope I misunderstand what you mean by custody, Master Windu. It smacks of treason.

MACE WINDU: You’re under arrest.

PALPATINE: Really, Master Windu, you cannot be serious. On what charge?

MACE WINDU: You’re a Sith Lord!

PALPATINE: Am I? Even if true, that’s hardly a crime. My philosophical outlook is a personal matter. In fact-the last time I read the Constitution, anyway-we have very strict laws against this type of persecution. So I ask you again: what is my alleged crime? How do you expect to justify your mutiny before the Senate? Or do you intend to arrest the Senate as well?

_{^{Revenge of the Sith: Official Novelisation}}

That being said, they also have evidence (from Anakin's testimony about Palpatine's admission) that Palpatine has been consorting with the enemy, so their decision to confront him was entirely valid. He's been committing treason and their intention was to arrest him for trial by the Senate rather than simply assassinating him.

MACE WINDU: In the name of the Galactic Senate of the Republic, you are under arrest, Chancellor.

MACE WINDU and the other JEDI ignite their lightsabers.

PALPATINE: Are you threatening me, Master Jedi?

MACE: The Senate will decide your fate.

_{^{RotS: Original Screenoplay}}

Without having a copy of the Republic Code of Justice in front of us, we can be reasonably certain that any decisions made in this matter would be political in nature, rather than being decided by recourse to the criminal courts, but their decision to arrest him immediately was a valid and proportionate one (and entirely legal), given the immediacy of his contact with the enemy and the severity of his crimes.

Answer 2

I can provide an answer for the Legends continuity.

Windu and his fellow Jedi were authorized by Yoda (per the Revenge of the Sith novelization) to carry out the arrest of the Supreme Chancellor. However, since they showed up at Palpatine's private office without any Senate guards or security forces in tow, nor with any apparent warrant or legal cause for Palpatine's arrest, it's quite possible that they were technically committing treason by forcibly deposing the Chancellor even if they were doing it with the Republic's best interests in mind. Without knowing the full extent of the Jedi Order's jurisdiction in arrests et cetera as applied to high-ranking Republic political figures, Palpatine's accusation of treason may have been legitimate or only been for his office's security feed to further push his narrative that the Jedi attempted a coup.

The Republic Commando books by Karen Traviss describe several "Contingency Orders for the GAR", two of which are Order 65 and the infamous Order 66. Order 65 reads as follows:

Order 65: In the event of either (i) a majority in the Senate declaring the Supreme Commander (Chancellor) to be unfit to issue orders, or (ii) the Security Council declaring him to be unfit to issue orders, and an authenticated order being received by the GAR, commanders shall be authorized to detain the Supreme Commander, with lethal force if necessary, and command of the GAR shall fall to the acting Chancellor until a successor is appointed or alternative authority identified as outlined in Section 6 (iv).

According to the Contingency Orders, it was legal to remove the Supreme Chancellor from office, but only with authorization from the Senate or the Security Council, neither of which was given when Windu attempted to arrest Palpatine. Taking matters into their own hands, despite Windu's conviction that Palpatine "[controlled] the Senate and the courts" and the Jedi's status as commanding officers in the GAR that would have allowed them to temporarily take command in the absence of the Chancellor, would probably count as a treasonous action under these circumstances.