The debate about Brexit since the referendum has been dominated by two arguments. One has been economic. The sky has not fallen in so Brexiteers say they are vindicated, while Remainers say we haven’t left yet. The other argument has been about the negotiations, which have been long and complicated, and have not yet reached the toughest aspects. Remainers say they are vindicated and Brexiteers say Europe is persecuting us.

But there is a third argument about Brexit, absent until now. It is about global politics and national security. It needs to come centre stage before it is too late.

The reason for this shift in perspective is simple: the biggest change since the referendum has been the election of President Trump. His redefinition of American interests, and the consequences for world order, at a time when liberal democracy is in retreat, are a game-changer. In the space of 15 months, the administration has shaken the assumptions of global politics - about friends and enemies, risks and rewards, values and interests. This is plenty reason to reconsider the fateful step of leaving Europe’s political alliance.

The forthcoming appointments of Mike Pompeo as Secretary of State and John Bolton as National Security Advisor only dramatise the stakes. Both are men who have made their names by advocating unilateral military answers to diplomatic problems. The global political climate is about to become more dangerous.

This would be important for Britain at any time. But this is not just any time. Whether President Trump lasts one term or two (or less), his impact will be long lasting, because he has helped define a new balance of power in the world. America is seeking a smaller role in the world when, and so, others are seeking to fill the vacuum. The world’s autocracies – from China to Russia to Turkey to the Gulf to Venezuela – are newly confident and clear about their power and license. They are making their plays. And that changes the calculus for a medium-sized country on the edge of Europe.

For 70 years, America has been an anchor of the global order. Often reluctant, and sometimes erratic (and wrong), America has nonetheless borne the burdens of global leadership with strategic care for the balance of power. No more.

Where the Truman Doctrine set out to limit the spread of communism, the Marshall Plan sought to rebuild democratic allies after the Second World War, and the end of the Cold War reunited Europe, the Trump Doctrine proceeds from a different profit and loss account. The consequences are already there for all to see.

Withdrawal from the Trans-Pacific Partnership has made China the leading power in Asia. Unilateral trade tariffs threaten to put the integration of the global trading system into reverse. The embrace of deal-making, replacing a bias for democratic advance, has given leeway to dictators. And then there is Russia, smelling leverage and license in the administration’s assault on its own institutions at home and alliances abroad.

The successful campaign to drum up Western support for diplomatic measures against Russia over the Salisbury attack only goes to show how important are the ties and institutions that bind liberal democracies together. It is also reasonable to say that President Trump’s failure to mention, never mind criticise, the attack in a phone call with President Putin takes British national security into uncharted waters.

Richard Haas, of the Council on Foreign Relations, says the US is now an “abdicationist” power. He means that the Trump administration is breaking a foundation of American foreign policy since the Second World War: the idea that America’s international alliances and global engagement are the foundation of America’s interests, and are a positive sum game, in which there are “win-win” solutions, whether on security or trade or climate. The consequence is uncertainty at best and dangerous vacuum at worst.

President Trump did not create the refugee crisis, nor the climate crisis or the North Korea crisis, nor even the crisis of inequality. But his actions threaten to exacerbate them. And when it comes to Iran he inherited a situation of radically diminished tension which he is threatening to upend. With the props of world order – institutions, laws and practices built after the second world war – under unprecedented duress, we are entering the most dangerous period in international relations in three generations.

The real danger is that small and medium-sized states, like Britain, will increasingly be at the mercy of the interests of larger powers. For a small example just look at the recent report of the trouble Canada and Mexico are having to get the Trump Administration to agree to warning labels on food in the renewed NAFTA trade agreement.

It is little wonder that Liam Fox should say he doesn’t want a “political” Brexit. He has smelt the coffee. He can see the dangers of Britain losing its place at the European top table. But it is utterly astonishing that he should think that Brexit can proceed without political consequences.

Brexit is, by definition, a political act. It removes Britain from the political alliance that has magnified our power and influence over the last forty years. From Ted Heath onwards, Prime Ministers have used our place in Europe as a multiplier of our power. And contrary to claims made since, Heath was clear when we joined in 1973 that this was one purpose of joining, to be part of a political project. It has been a mistake to downplay that.

Brexit is bound to have political consequences – for the international order and for Britain. Brexit takes a brick out of the western alliance and the international order at a dangerous time. And for Britain it weakens us when we can least afford it.

It is obvious that the only language Russia will understand is one delivered by a united West that is able to mobilise international action, including from new Russian allies like Israel and Gulf countries. In curbing Russian rogue state behaviour, Britain needs Europe.

We know from the current Prime Minister that we also need Europe to deal with security threats, yet we also know from the Head of Europol that Brexit makes that harder. And we will get less of a hearing in the cacophonous and extreme hothouse of US politics isolated from the economic and political clout of France, Germany and the rest of Europe. That matters to the global stability on which British prosperity in part depends.

The Prime Minister spoke at the Munich Security Conference in February about the need for cooperation on security and defence policy after Brexit. That’s good. But the proposed measures were a minimum of damage limitation for the unilateral political disarmament that Brexit represents. Observer status in meetings or special mechanisms for cooperation cannot make up for the structured and ongoing shared action that has become the norm over the last 40 years.

The great danger is that none of this is factored into debate on the final Brexit deal. The Brexit deal is due to be presented to Parliament by October this year. But the withdrawal agreement is focused on the divorce bill and the rights of EU citizens. It does not account for global politics.

The transition agreement – more like a standstill agreement – does not cover it. And the addendum to the legal text, a political declaration about the future relationship, is likely to fudge the big questions.

The Conservative and Labour leaderships united in an appalling act of short-sightedness to trigger the two year Article 50 process in March 2017, way before either had worked out the basics of what Brexit should mean. Now they are uniting to say that it is too late to think again. That isn’t good enough. It leaves Parliament to speak up for the foundations of our security and our place in the world – to warn about the dangers ahead and to give people the chance to have a final vote on the final deal. John Major’s proposal for a free vote would aid this process. But the end game in order to avert the damage of Brexit is going to need to be in the hands of the people.

In 2016, the foreign policy environment was benign enough to defy David Cameron’s attempts to bring it into the referendum debate. His argument about the position of Britain in the global balance of power after Brexit was mocked as a prediction of World War 3.

Today it is no longer possible to be so sanguine. The values of liberal democracy are in retreat. The threats to peace around the world are more acute than ever. Britain’s role as a stabilising power in the global system has rarely been more needed. This is no time to give the forces of revanchism a free hit.