Just suppose the Russians invaded Norway: it would not be hard.
The border right at the top of Norway is just under 200 kilometers that divide the Pechengsky District of Russia and Norway’s Sor-Varanger.
A Russian night air and seaborne bombardment followed by a night armoured and guards armour assault should do it by the following weekend.
To tighten it up, a Russian submarine operation plus a tidy-up after the morning battle damage assessment would change the whole security position of one of NATO’s vulnerable borders.
The NATO rules declare that the rest of NATO has to then kick the Russians back into Russia.
But would they? And anyway, could they?
So Exercise Trident Juncture is currently underway in Norway to see what would happen if the Russians attacked the northern European country.
This is the biggest NATO exercise since the end of the Cold War.
And it is not just another NATO end-of-year outing. Given the miserable state of what used to be called East-West relations in the years of the Cold War, Trident Juncture has major political as well as military importance.
There are 29 nations in the North Atlantic Treaty Organisation (NATO). All 29 have signed up for Trident Juncture and so have Russia’s neighbour Finland and neutral Sweden.
That is political solidarity with all three services, role-playing. More than 60 ships, 150 aircraft, 50,000 sailors, soldiers, airmen and women.
It has a single ambition: to prove NATO’s readiness to test the sovereignty of an ally after an act of aggression.
But first, best not mention Ukraine nor Crimea.
Second, this is an exercise so none of the Allies will face real military nor diplomatic nor domestic pressures to stay out of any rescue mission.
However, the subnotes of Trident Juncture in its backroom stage do have a what-if section.
To start with, who would turn up for war in the first place? More realistically, who would politically and militarily be in a state to do so?
Some of the small NATO members do not have the resources to even get into position and certainly lack the political will.
So if Trident Juncture has proper value, it has to be realistic in the political sense.
The real value will be an assessment of how quickly NATO’s military can move and keep moving.
Is the integrated command structure in all three services workable as it says it is on paper? Can reinforcements reinforce? Can different military commands integrate?
At what point, if at all, would it be realistic to write in a request for nuclear weapon release?
And, until recently, an unasked question: if Russia moved on Norway, would the United States - with its huge tribute-service European deployment - take part in a rescue operation?
Might an early morning tweet from a golf course simply declare: “Not coming. You don’t pay your 2%".
Orange and Blue Forces, South and North forces are wargaming images, but increasingly there is a need to know the wash-up answers to the obvious: did everyone turn up as they said they would? Did they commit themselves and were they any good?
It's just an exercise, but look around at the world in which NATO operates today and you will know Trident Juncture could easily one day be the real thing.
Christopher Lee is the Forces Radio, BFBS Defence Analyst. He can be heard every week on the only radio programme devoted to discussing matters of defence and security, Sitrep. To listen head here, to download click here.