The DUP have been very worried about the Army lately. The Government has a plan – Army 2020 – which envisages a reduction of 20,000 regular troops over the next seven years, due to budget cuts.
Ministers want to boost the reserve forces and use them "routinely, rather than in extreme circumstances", including long tours of duty overseas.
Army 2020 requires 30,000 trained reservists, complementing 82,000 regular personnel.
Rebel Tory MPs say the plan won't work.
They joined forces with the DUP on an amendment to the Armed Forces Bill last week that sought to delay any reforms until ministers produced a report to assess the "viability and cost-effectiveness" of the proposals.
The DUP's backing added pressure on the coalition – the potential rebellion could not be portrayed as the usual Tory malcontents.
The Government worried it could be defeated, so during the debate, loyal MPs piled on the pressure.
The rebel plan "will send a devastating message to the reserve forces", said Julian Brazier. Others warned any delay in recruitment "will cause confusion in the TA".
Ministers, anxious to avoid a climbdown, recalled that the DUP can, at times, be amenable to pork barrel politics – and rightly so.
With eight MPs, they are the only local party with enough votes in the Commons to be an effective caucus.
Northern Ireland's voice can, at times, struggle to be heard in a chamber of 650 MPs, so it is right for the DUP to cut a deal when ministers come knocking. It did not help the rebel cause that its leader was John Baron, whose disloyalty to the prime minister is so pathological that it repels many of those who actually agree with him.
The DUP said very little during the debate.
Jim Shannon, who does like to share his views with the House when he can, pointed out that recruitment in Northern Ireland to all the services "has met levels higher than in the rest of the United Kingdom".
Could a deal be done on recruitment?
Moments before the vote, Jeffrey Donaldson (above) intervened on Defence Secretary Philip Hammond to point out that Northern Ireland reserve units are at 100% recruitment capacity.
"Why not extend and raise the ceiling for recruitment in successful areas?" he asked.
Hammond replied: "I can tell the right honourable gentleman that, although we have not publicised it, we have increased the recruiting cap on units in Northern Ireland to 115% of liability."
This was news to MPs, but there was more to come. "A review is currently underway to look at trade skills available in Northern Ireland," Hammond added. "Most of the reserves recruiting is trade skills-specific. If we find that pools of additional trade skills are recruitable, we will consider locating additional units in Northern Ireland to tap into them."
The DUP spurned the amendment they had signed and voted with the Government. The rebellion had collapsed.
In spite of Labour support, the amendment was defeated by 306 votes to 252. They may have a reputation for intransigence, but the DUP can cut a deal when it suits them.