The DUP are in an odd place at the moment. They can't, in fairness, be entirely surprised to find themselves in that place, because they have been warned for some time that "arrangements" between "Ulster" unionism and the Conservative Party rarely end well.

I think they must have known that this latest crisis was coming; indeed, they have been firing their own shots over Theresa May's bow for almost a year. And yet she still faced them down and let them down.

Their immediate reaction was to lash out. What else could they do? Not only had they been "done over" by the Prime Minister (which isn't unusual, of course; Heath, Thatcher and Major have done the same sort of thing), they had been "done over" by a Prime Minister they have propped up for almost 18 months.

This was a Prime Minister who knew that "doing over" the DUP risked her own career and the stability of her Government. She did it anyway.

A year ago, Arlene Foster boasted that the "Union was secure". Today, she believes that same Union is in jeopardy.

Is she right? In November 1985, all of unionism believed that the Anglo-Irish Agreement was "a gross betrayal - the beginning of the end of the Union as we have known it". In 1998, the DUP described the Good Friday Agreement as "a one-way ticket to Irish unity".

But 33 years on from the Anglo-Irish Agreement and 20 since the Good Friday Agreement, Northern Ireland is still an integral part of the United Kingdom. The "constitutional guarantee" - that Northern Ireland will remain a part of the UK until a majority decides otherwise - remains in place.

And it's worth bearing in mind that the DUP have insisted, for a long time now, that, irrespective of Brexit, unionism has nothing to fear from a border poll. Yet, it now looks as if the DUP fear that they are facing the "hardest" Brexit of all: a deal which avoids a hard border, but threatens the constitutional integrity of the United Kingdom.

I noted last week that there was no immediate, or inevitable, threat to the Union. That remains my view. But treating Northern Ireland differently - and leaving open the possibility that its particular relationship with the EU would continue to be different to that of the rest of the UK for a long, unspecified period - will, at some point, raise very difficult questions about the precise nature of the relationship between Northern Ireland and Great Britain.

Put bluntly, it raises the question of whether or not Northern Ireland's formal link to the UK would be compromised; or, worse still, damaged irreparably.

The answer to that question isn't contained in the wording of the Withdrawal Agreement. That's why the DUP and broader unionism are concerned.

There clearly isn't the same sense of fear and abandonment that I remember from November 1985; but the Conservative Party, Mrs May, her Cabinet and the entire House of Commons must not assume the lack of fear and lack of on-the-street protest means unionism doesn't need reassurances and guarantees.

I have never had a problem with a final deal that recognises what Brussels, London and Dublin describe as the "special circumstances of Northern Ireland"; but that deal and those "special circumstances" must also embrace the constitutional rights and identity of "Ulster" unionism.

One very senior Conservative MP told me that Mrs May had finally decided to "stand up to the DUP". But given the nature of unionism here, it means that the other unionist parties will rally round the DUP if they genuinely believe there is a substantial threat to the Union.

Even those parties who blame the DUP for "getting it so wrong" with the Conservatives, and who mock them over the RHI revelations, will stand shoulder-to-shoulder with them in the name of the Union.

They will jointly attack May and the Irish government. And if - and I certainly wouldn't rule out the possibility - a deal which includes the present "solution" to the Irish/Northern Ireland problem is passed in the Commons, then I would expect to see the sort of coming together of unionism that we haven't seen since Sunningdale and the United Ulster Unionist Coalition.

I also think the same coming-together is likely in the event of a "no deal".

All of this raises an old question, one I have asked many, many times down the years: why do unionists seem to be on the "losing" side so often? There is no point blaming the Irish government for stirring the pot, or planning a land-grab. They are doing exactly what the UK Government would do if the roles were reversed; prioritising, protecting and promoting their own interests.

And, in fairness, Brexit is not a problem of their making; nor is it a problem they ever wanted landed with.

Also, they're not after Irish unity anytime soon. Who in their right mind would push for unity when there is so much uncertainty and, potentially, so many angry unionists?

If you thought the Brexit debate is complicated and fractious, just imagine a full-blown Irish unity debate.

As I noted above, May is doing what Heath, Thatcher, Major and Blair have done: which was once described to me as "being far too impartial on the issue of the integrity of the Union when it comes to Northern Ireland". In other words, they were prepared to face down "Ulster" unionism for the sake of a deal which they believe to be in the UK's national interest.

And when they have chosen that course of action, there hasn't been a backbench rebellion in favour of "Ulster" unionism.

It's worth bearing in mind that a majority of May's Cabinet supported the Withdrawal Agreement; and if that agreement goes to a vote in the Commons, it's likely that a considerable majority of Conservative MPs would support it, along with the provisions for Northern Ireland.

That lack of support is an ongoing problem for "Ulster" unionist parties.

The DUP - who were scathing about the UUP/Conservative UCUNF project a few years ago - may have believed that Mrs May's reliance on them for her own survival would have prevented her from "betraying" them. It didn't.

She must have been aware of the inevitable scale, nature and consequences of their anger; yet she went ahead.

Her "precious" Union and unionism doesn't seem to embrace the equally "precious" Union and unionism of the DUP and broader unionism across Northern Ireland.

That fact, that reality, should probably worry the DUP more than anything else which has happened over recent days.

Belfast Telegraph