Having boasted last November that their relationship with the Conservatives meant the Union was secure, the DUP has now concluded Tuesday’s agreement undermines the "constitutional integrity of our precious Union".

They will vote against it, even if it means bringing down the government. IT is no surprise it came to this. Previous relationships with the Conservative Party have ended badly: as Edward Carson noted in 1921, “What a fool I was. I was only a puppet, and so was Ulster, and so was Ireland, in the political game that was to get the Conservative Party into power.”

The link between unionism and the Conservatives has its roots in the Home Rule Crisis in the mid-1880s, when, in response to Irish Nationalists holding the balance of power, what we now understand as ‘Ulster unionism’ (which was always different from broader, pan-UK unionism) emerged as a separate political/electoral identity.

But even after Carson’s criticism the Ulster Unionists remained a fully integrated part of the Conservative Party between 1922 and 1972, a number of their MPs serving in government and holding senior positions within parliamentary and party committees.

When Margaret Thatcher became leader in February 1975 she was viewed by unionists as more of a unionist than Heath

The end of the formal link can be traced to a speech by Prime Minister Edward Heath in November 1971: “Many Catholics in Northern Ireland would like to see it unified with the South. That is understandable. It is legitimate that they should seek to further that aim by democratic and constitutional means. If at some future date the majority of the people in NI want unification and express that desire in the appropriate constitutional manner, I do not believe any British Government would stand in the way.”

The UUP, already under pressure internally and externally, interpreted the speech as a direct attack.

Four months later, Heath prorogued Stormont, and UUP MPs refused the Conservative whip. Matters got worse in October 1972 when the government endorsed mandatory power-sharing and an Irish dimension; leading former UUP leader James Molyneaux to describe it as a "gross betrayal" of unionism and the Union. The Conservatives also supported the Sunningdale Agreement in December 1973; a decision that led to the biggest breach between Ulster unionism and Westminster since 1912-14.

When Margaret Thatcher became leader in February 1975 she was viewed by unionists as more of a unionist than Heath. They also believed she was in awe of Enoch Powell (elected in South Down in October 1974) and would be steered by his integrationist arguments.

She wasn’t. As soon as she became Prime Minister she announced a talks process, which the UUP ignored; followed by the 1982 "rolling devolution". Assembly, which both the SDLP and SF boycotted. Thatcher was also responsible for the 1985 Anglo-Irish Agreement, viewed across unionism as the "ultimate betrayal". Crucially, as with Sunningdale, there was no significant pro-Ulster unionist rebellion by the Conservative parliamentary party.

Given that history it was a brave decision by some people--mainly from middle-class unionist areas--to establish ‘model’ Conservative Associations. They described "Ulster" unionism as insular, sectarian and damaging Northern Ireland’s constitutional future, arguing that fully integrated NI Conservative Associations would provide a ‘broader-based, non-Orange unionism’ into which voters (particularly middle-class Catholics) could buy.

This is not 1912. Nor is it 1985. Unionism has changed. Their majority has diminished

The 1989 Conservative Conference gave the green light and in the 1992 general election the party did quite well in a few places. That was their high point. Within five years it was clear that they were a political and electoral irrelevance, ignored by Central Office, briefed against to both the UUP and DUP, and struggling to muster 1% support.

They weren’t helped in late 1993, when John Major and Secretary of State Patrick Mayhew had to row back from previous denials that they were facilitating back-channel negotiations with Sinn Fein. Local unionist parties were also very concerned with the tone and direction of the Downing Street Declaration in December 1993, particularly the line that the government had "no selfish strategic or economic interest in NI".

In 2007, when the UUP and Conservatives announced an electoral pact (David Cameron was looking for seats anywhere and the UUP needed something to give it an edge over the DUP) the DUP was scathing: "When it comes to Northern Ireland the history of the Conservative Party is not one which would give confidence to any true unionist."

Ian Paisley took delight in listing the "serial treachery" of Heath, Thatcher and Major. Today, there are many in the DUP who are wondering if Theresa May should be added to that list. She, of course, wouldn’t view her actions as treacherous; but, like her predecessors, she will always have interests that are bigger than those of either the UUP or DUP.

There is a lesson in this for all of unionism, the DUP in particular. Stop believing the Conservative Party is the only party worth reaching out to. Spread yourself; talk to other parties in London and Dublin. Don’t regard them as your natural enemies. Almost a century on, reread and heed Carson. The megaphone and public threats will not sort the present mess.

This is not 1912. Nor is it 1985. Unionism has changed. Their majority has diminished. Above and beyond everything, though, remember that NI will only leave the Union when a majority votes to leave the Union. Focus on that.

Alex Kane is a commentator based in Belfast. He was formerly director of communications for the Ulster Unionist Party