The first months of Donald Trump’s presidency have been marked by extraordinary chaos and disruption. The administration has begun to come apart at the seams, above all because of the president’s own behaviour and incompetence. Every day brings a new diversion. But the appointment by the US justice department of a special counsel to investigate Russian interference in the 2016 presidential election is a clarifying act. For the administration it is a watershed moment. It may also, perhaps, prove to be a Watergate one.

Special prosecutors are lesser but still powerful versions of judge-appointed independent counsels such as Archibald Cox during the Watergate investigation itself and Kenneth Starr in the Bill Clinton era. Congress found them just too independent and the law authorising them lapsed in 1999. Robert Mueller is in the job thanks to the deputy attorney general, Rod Rosenstein, who took on the issue after his ideologue boss Jeff Sessions rightly recused himself, and has some of the appearance of a political appointment. But it has been welcomed by senior Democrats and Republicans alike. That the appointment was made in the face of White House insistence that it was entirely unnecessary, and in the midst of yet another “worst week so far” for the president, reflects well on Mr Rosenstein and the justice department. The decision is a sign that constitutional principles and ethical norms survive within the federal government in spite of Mr Trump’s utter disregard for them in so many ways. It will reshape politics on Capitol Hill too.

The appointment order is cast extremely wide. Mr Mueller’s principal mandate is to investigate “the Russian government’s efforts to interfere in the 2016 presidential election”. But the terms also instruct him to pursue “any matters that arose or may arise directly from the investigation”. This brings the whole relationship between Mr Trump and the Russian government since the election into the investigation’s orbit. It also permits Mr Mueller to pursue the issues that have roiled US politics for the past two weeks – the dismissal of the FBI director James Comey, the possible taping of White House interviews, and an alleged obstruction of justice by Mr Trump by pressing Mr Comey to stop a probe into the former national security adviser Michael Flynn.

Special prosecutors have wide powers to follow an investigation wherever it leads Mr Mueller can subpoena papers and tapes, and go to the courts, as Mr Cox did at the time of Watergate, to require cooperation. He can interview anyone up to and including the president, as Mr Starr did at the time of Whitewater. He can bring charges against anyone caught up in the process. As a former FBI director himself, Mr Mueller will have enormous authority to shape the endgame.

Mr Comey has been asked to give evidence on Capitol Hill next week. This is a reminder that, as in Watergate, senate and house committees may continue to play a central role in maintaining the momentum of inquiries at the same time as Mr Mueller continues his investigation. American government has not necessarily embarked on a journey that will lead to presidential impeachment or resignation. But such possibilities are now serious options. These are facts not fakes, and the Trump presidency will stand or fall in the light of them.