By voting against the Government the DUP has sent David Cameron a powerful message that he cannot take their support for granted.
The Prime Minister may have assumed that the DUP would also take a strong line on Syria and that the votes of its eight MPs were in the bag.
If so, he will have been disabused when he rang Nigel Dodds, the DUP's parliamentary leader, to rally support earlier this week.
The DUP backed a Labour amendment which would have left the way open for the use of limited force, but only if the UN received "compelling evidence that the Syrian regime was responsible for the use of these (chemical) weapons".
This was a defensible position in its own terms, but by voting with Labour against the Tories the DUP has increased its parliamentary clout.
There may be more tight votes, perhaps even a breakdown of the Liberal Democrat/Conservative coalition before the 2015 election, and after that the expectation is that no party will have an overall majority.
The support of smaller parties like the DUP could be at a premium and, by showing independence, it has increased its bargaining power.
It appealed to Theresa Villiers, the Conservative Secretary of State, to intervene in marching disputes and overrule the Parades Commission.
The DUP hopes to have nine or 10 seats after the next Westminster election, adding East Belfast and possibly North Down to its current eight.
That is a very useful number and the party has discussed how it would act if there is a tight result. DUP sources say that there is no prospect of going into coalition with the Conservatives; that would make it a hostage to fortune on issues like gay marriage and abortion.
What might be possible would be an external support arrangement in which the DUP would agree to support a vulnerable Government in confidence votes in return for delivery on issues of concern to it.
In the meantime, Mr Cameron and Ms Villiers will have to think more carefully about crossing Mr Dodds or his colleagues in the future.