Why was a special court created to try Johan van Oldenbarnevelt?

A special court was created to try Johan van Oldenbarnevelt because there was no existing federal judiciary to handle the charges brought against him.1

The conflict arose from a fundamental disagreement regarding legal jurisdiction. Oldenbarnevelt maintained that, as a civil servant of the sovereign province of Holland, he was accountable only to that province's judiciary. Conversely, his political opponents sought to try him for felony and treason against the union of the United Provinces.12

To resolve this impasse, the States General instituted an extraordinary tribunal ad hoc consisting of 24 judges. This court was composed of individuals who were not only political rivals but, in many cases, personally antagonistic toward Oldenbarnevelt. Ultimately, he was condemned by this special body and executed in 1619 for the "subversion" of the country's religion and policy.12