On 4 April 1770, the Brittany affair entered its final phase with the trial of the duc d'Aiguillon before the Parlement of Paris. He stood accused of abusing his authority whilst serving as commandant in Brittany, and of attempting to procure witnesses to testify against his great enemy, the procureur général of the Rennes Parlement, La Chalotais. The trial of d'Aiguillon commenced amidst much ceremony, but a definitive verdict was never reached. On 27 June, the king held a lit de justice at which the duc was declared innocent of the charges against him. The Parlement refused to accept that a peer could be exonerated by force of authority, and the ceremony marked the beginning of a destructive conflict with the government. Determined to end the affair with a victory for the crown, the chancellor, Maupeou, extended the dispute into a constitutional battle of the first order by publishing an edict curbing the powers of the Parlement. Undaunted, the judges refused to recognise the new law and, with neither side willing to compromise, Maupeou was forced to exile the magistrates in January 1771. It was the first stage of an ambitious reform of the judiciary which contemporaries christened ‘Maupeou's revolution’.

Not surprisingly, these momentous events have long attracted the attention of historians. Perhaps the most durable interpretation of the crisis has concentrated upon the broader theme of a reforming monarchy overcoming the reactionary opposition of the Parisian magistrates. The evidence to support this assertion is not convincing, at least in the case of the Parlement of Paris, but that does not necessarily undermine the thesis of Maupeou as a reforming minister.