Whenever Theresa May signed up to a confidence and supply deal with the DUP following the Westminster election in 2017, she must have known that at some point the priorities of both sides would no longer converge.
The prime minister had been left with few options after her dreadful showing in that snap election and needed the ten DUP MPs to keep her government going.
For their part, the DUP drove a hard bargain, securing a £1 billion cash injection for Northern Ireland and for a while all seemed harmonious.
However, such forced political relationships rarely run smoothly and we are at the stage where suspicion and distrust is poisoning the atmosphere.
There was a wobble in December when the prime minister had to make a humiliating climbdown on the backstop wording in the face of DUP anger.
That episode will have told Mrs May that the largest unionist party is not afraid to flex its muscles if it believes there is any movement outside its 'blood red lines'.
In recent days we have seen the rhetoric stepped up as the crunch time nears when the prime minister will either find an agreed outcome or walk away without a deal, something that is generally accepted would be disastrous.
The DUP this week threatened to vote against the budget later this month if Mrs May concedes to a deal that treats Northern Ireland differently to Britain.
And to show that it is serious, the party abstained from a vote on an amendment to an agriculture bill on Wednesday night.
This was a warning shot which did not impact on the overall outcome but the stakes will be much higher if the DUP helps to defeat the government on the budget, which could lead to a no-confidence motion.
Sammy Wilson has made it clear the party is prepared to see Mrs May jettisoned if she does not take their concerns on board.
The party seems to think the worst that could happen is a change of Tory leadership, but that ignores the real possibility of another election which could result in Jeremy Corbyn arriving in Downing Street.
Are the DUP really prepared to take that chance?
It is a big gamble and obviously the stakes are high as we enter the end stages of negotiations.
Theresa May, who met key cabinet colleagues last night to rally support, has some tough decisions to make in the coming days and weeks.
The DUP is playing its strongest card, but the prime minister, in the middle of finely balanced negotiations, may not appreciate being held to ransom by the party.
