Why have historians described the trial of Johan van Oldenbarnevelt as unfair?

Historians view the trial of Johan van Oldenbarnevelt as fundamentally unfair due to several critical procedural and political irregularities:

• Lack of Jurisdiction: Oldenbarnevelt maintained that as a civil servant of the sovereign province of Holland, he was accountable only to that province’s judiciary. His opponents, however, insisted on trying him for felony against the union, despite the fact that no federal judiciary existed to hear such a case.1
• Biased Tribunal: To bypass the lack of a federal court, an extraordinary tribunal was created ad hoc. This body of 24 judges included individuals who were not only political adversaries but also personally antagonistic toward the defendant.1
• Questionable Qualifications: Many of the judges appointed to this extraordinary tribunal were not qualified lawyers, undermining the legal legitimacy of the proceedings.1
• Political Motivation: The trial was the culmination of a power struggle between Oldenbarnevelt and Prince Maurice. Although public opinion had been heavily propagandized to expect a conviction for high treason, the court ultimately condemned him for the vague charge of subverting the country’s religion and policy.1