If Russia invades Ukraine this week, thus provoking the first full-scale war in Europe between two states since 1945, Britain will support the aggrieved party, as it should.

The full suite of instruments outlined on this site by Garvan Walshe should be deployed in response: that’s to say, information, economic pressure, sanctions that target individuals, and military aid.

That help will stop short of the deployment of our troops in the Ukraine, but Vladimir Putin may nonetheless wage asymetric conflict against western countries, through cyber attacks, for example.

This is all that needs to be said about the short-term, at least for the moment, but the medium-term is a different matter.

For the crisis in Ukraine raises profound questions that the main political parties have been unwilling to answer about our foreign policy, our defence posture – and the future of NATO itself.

Let me illustrate them by outlining two potential and different strategic approaches to Russia during the years ahead.  Both would have the same starting-point: that its corrupt autocracy is necessarily antagonostic to liberal democracies.

With “loyalist security forces, a subservient judiciary, a controlled media environment, and a legislature consisting of a ruling party and pliable opposition factions, [Putin] is able to manipulate elections and suppress genuine dissent.”

“Rampant corruption facilitates shifting links among bureaucrats and organized crime groups.” Such is the verdict of Freedom House, amplified in British memories by the regime’s Novichok attack on our own soil.

The first approach would be transatlantic in flavour – at least for as long as America maintains its present stance on Russia – and be marked by an idealistic projection of western values through military strength.

It would be consistent with the further enlargement of NATO, including membership for Ukraine, Georgia and Bosnia & Herzegovina.

Which would necessitate member states other than the US, Britain, Greece and Poland meeting the NATO minimum spending requirement of two per cent of GDP on defence.

Such a policy would doubtless write off consequent Russian movement further towards China’s sphere of influence as a development that will happen anyway.

In effect, we would be committing to treating an attack on Ukraine, say, as an attack upon us under the terms of Article Five of the NATO charter – and it would be necessary to be upfront with voters about the consequences.

This posture would have big implications for our defence policy, suggesting armed forces less orientated to projecting power outside the European theatre.

The second approach would be more European in tone, not unlike that currently being followed by Germany and, more prominently, by France (or at least by Emmanuel Macron), and stress realism.

It would recognise that there is little likelihood of many NATO member states spending the required minimum, accept that there are limits to our own defence budget, and view the public as essentially unsupportive of distant wars.

It might well conclude that it is sensible to concentrate our military resources in eastern Europe to support allies to whom we are already committed.

However, it would be resistant to the enlargement of NATO, refusing membership to Ukraine, Georgia, Bosnia & Herezegovina and almost anyone else who may apply for it.

It would toil to keep Russia within the western orbit as far as is possible, and seek to prevent it from lining up with China in a working alliance of non-democracies.

And while the first approach would be hard to reconcile with working with Russia as a partner on climate change, this second would see no such difficulty.

You will already have worked out that successive governments have all but led us to the position of committing to the first approach but with the means to deliver only the second.

Politically unengaged voters may not have been so quick off the mark.  They are also likely to regard NATO, insofar as they have heard of it at all, as a pillar of UK defence policy and a fact of British political life.

However, the security and defence choices that this Government and its successors must make are bound up with the future of the alliance of which Britain has been a member for the best part of a century.

This is not the time to be debating the matter on the floor of the Commons, but I hope that our defence and diplomatic establishments and others are thinking about it very hard indeed.

NATO was the cornerpin of our security during the Cold War, having as it did a clear vocation to counter the Soviet Union’s totalitarian aggression.

Since the collapse of communism, its membership has rolled into eastern Europe, and its influence well beyond.  For example, Tajikistan is now a “partner country”.

When I last looked at an atlas, Tajinkstan was some distance from the north Atlantic. The relationship presumably, like that with Kazakhstan, the Kyrghyz Republic, Turkmenistan and Uzbekistan makes sense in the Russian context.

This is what the Putin regime is seizing on in its complaints about encirclement.  But hang on a minute.  What has Colombia’s status as a NATO “global partner” got to do with Russia?

Answer: nothing much.  NATO presumably has a foothold in the “war on drugs”.  As it does in the West’s collective response to China, if Australia and Japan’s status as partners is anything to go by.

Furthermore, the NATO country with the second highest proportional GDP spend on defence is Greece – in preparedness for potential hostilities with another NATO member, Turkey.

Its member states have otherwise mostly proved resistant to Putin’s overtures.  And what one man might see as incoherence, in terms of the alliance taking military action, is another man’s flexibility.

For NATO not only invoked Article Five after 9/11, but has sometimes taken action outside its terms – for example, in Kosovo.

Perhaps the ambiguities of NATO are inevitable in an age of multiple threats to western democracies, a challenge recognised by the Government’s Integrated Review.

But there seems to me to be a distinct possibility that, in relation to Russia, we may eventually find ourselves with obligations to allies for which the electorate is almost completely unprepared.

And if the language of the late 1930s is appropriate – Ben Wallace has been complaining of a “whiff of Munich” in the air – Minister will have to provide the means to meet the end.

By 1939, remember, 48 per cent of all Government spending under Neville Chamberlain was going on defence.  Not that even the most aggressive upping of our present spend would require that much.

Nonetheless, we are in collective danger of drifting towards hostilities this week with habits, assumptions and reflexes that are out of date.