Today is the anniversary of the execution of King Charles I.
The critical point, so often missed, is that Charles was tried, as King. He was then executed, as King. He had not been deposed. His place had not been taken. He was brought to trial while still acknowledged as King, and remained King until the axeman's blade struck.
He was tried according to the Common Law of England for breaking the law. He was tried for having though himself above the law - for breaching the principle that the law applies to the common man as much as it does the sovereign. For example, Under Charles' reign, defendants were regularly hauled before the Court of Star Chamber without indictment, due process of the law, right to confront witnesses, and their testimonies were routinely extracted by the King and his courtiers through extensive torture. Sound familiar?
The English, and by consequence the Empire/Commonwealth, established that principle then, and have never deviated from it.
It is too bad that this lesson appears not to have been learned here in the US.