The White House national security adviser is one of the supremely important roles in any US administration. The adviser is the key voice in the Oval Office on international affairs. Presidents have often given the job to big figures, among whom Henry Kissinger was the biggest. Typically a national security adviser stays the course for a whole four-year term at the president’s side, thinking and talking in lockstep with the boss. None, until now, has held the job for less than a month, as Michael Flynn did before resigning on Monday night. Make no mistake, this is not some minor scratch to the Trump administration’s paintwork. It is a head-on crash affecting its whole intended direction of travel and its priorities.

Officially, Mr Flynn resigned because he misled officials, and in particular Vice-President Mike Pence, about the content of conversations that he had with Russia’s ambassador to the US before Donald Trump’s inauguration. Badly briefed, the officials and the vice-president then went public in his defence. “Unfortunately, because of the fast pace of events, I inadvertently briefed the vice-president-elect and others with incomplete information,” is how Mr Flynn put it in his resignation letter this week.

Yet this resignation wasn’t just caused by a piece of incompetent discourtesy to colleagues. It was caused by the Trump team’s appeasement strategy towards Vladimir Putin’s Russia, by the resulting mistrust between the Trump team and the US security agencies, and by the implications of Russia’s role in Mr Trump’s election. This resignation isn’t a Whoops; it’s a Wow.

Legally, it is an offence for a US citizen to meddle in foreign policy when not part of the administration. This was Mr Trump’s and Mr Flynn’s status when Mr Flynn spoke by phone to ambassador Sergey Kislyak in December. In practice, candidates and presidents-elect sometimes try to shape the international agenda in the run-up to taking power. In the most egregious case of all, Richard Nixon secretly conspired with the South Vietnamese to stop them going to the peace talks table in Paris in October 1968, a move that might have tipped the upcoming presidential election to the Democrat Hubert Humphrey. More than 20,000 Americans, as well as countless Vietnamese, were still to die in a war that Nixon successfully prolonged for his own political purposes.

Mr Trump’s motives for rapprochement with Russia are muddier. Yet Russia’s aims are clear, and its readiness to support him is proven. Russia wants the lifting of sanctions imposed by the Obama administration and its allies following the annexation of Crimea. Mr Trump and, perhaps, Mr Flynn have seemed ready to oblige. If that was indeed what Mr Flynn discussed with the Russians in December, it was a step too far, too fast and too dangerous. Happily, it will be harder now.

The policy has already put the president on a collision course with the US security agencies and diplomatic service, as well as with American voters, sections of his own Republican party and most Democrats. When she learned the details of the call from a phone tap, the acting attorney general Sally Yates, a Democratic appointee, informed the White House. But it was Ms Yates who was soon out of a job, sacked by President Trump when she queried his seven-nation entry ban. Mr Flynn, by contrast, was still being defended right up to the weekend. It was only the Washington Post’s reporting that finally skewered him.

Relations between elected governments and security establishments have often been fraught, even before the Iraq war debacle. Sometimes the potential for suspicion requires firm action by government. But politicians can get the relationship wrong too. Few issues are more toxic than Russia. Given the enduring scale of US fears about Mr Putin, it is not surprising that the election of a president backed by Russia and seemingly open to Russian demands causes alarm, particularly if the Russians have a hold over him. The implications for allies who share intelligence with the US, as Britain does, could be huge.

So much is potentially at stake that the official instinct on both sides of the Atlantic is still to be cautious. Inquiries are under way by the FBI and the senate. The immediate question is what Mr Flynn actually said to Mr Kislyak, and with whose authority. The deeper question is why Mr Trump should be apparently so willing, and in such reckless ways, to gift Russia something it wants so much. These are very deep and destabilising questions. Yet America and its allies must be ready to follow them wherever they lead.