Nic Robertson is CNN's international diplomatic editor. The opinions in this article belong to the author.

(CNN) Brexit is entering some make-or-break weeks. The contentious issue of the border between the Republic of Ireland and Northern Ireland is coming due.

Movement on all things Brexit -- be it the internal machinations of British Prime Minister Theresa May's government or direct talks with the European Union -- has been glacial.

Agreements arrive incrementally, often accompanied -- to quote Matthew 13:42 -- amid much "wailing and gnashing of teeth" to the point that many of us want to close our ears until the next phase of Brexit is done with.

But to do so this week would have meant missing some of Britain's constitutional crockery being banged around by those who should know better.

Some hardline Brexiteers on both sides of the House of Commons have been undermining Northern Ireland's Good Friday Agreement (also known as the Belfast Agreement), calling it "unsustainable."

The agreement has survived the tumult of 20 years of provincial point scoring. It ended 30 years of bloodletting. But now some appear prepared to see it sacrificed at the altar of a so-called hard Brexit.

It smacks of politicking at its most dangerous -- staking peace and stability as the price in a power play to hobble May.

The price for giving up on the Good Friday Agreement could be steep: It would likely invoke Ireland's wrath, cause a subsequent EU backlash and draw global opprobrium for bringing down one of the 20th century's most successful peace agreements. It would also put wind in the sails of those who dream of a united Ireland.

Northern Ireland's majority may still be some way from that conclusion right now, but the interventions of the hardline Brexiteers are stirring anger south of the border.

The escalation is happening for a number of reasons, chief among them the Democratic Unionist Party 's inability to evolve.

At the DUP's core is a desire to keep Northern Ireland an inseparable part of the United Kingdom. Right now it has the power to do that: Its 10 members of the British Parliament prop up May's government since she lost her majority in last year's general election.

Since its inception, the party has remained rooted in a past bound by traditions that it believes makes Northern Ireland British.

Yet DUP's politics are antithetical to mainstream mainland thinking. The party is staunchly against same-sex marriage, which is already legal in the rest of the UK following legislation by the Conservative Party under former Prime Minister David Cameron. Yet it now holds May's Conservative government hostage.

On December 8, May's Brexit negotiators provisionally agreed to a 15-page, 96-point joint report with their EU counterparts ahead of an EU summit that allowed the UK to move from phase one of the Brexit negotiations -- agreeing to the terms of Brexit -- to phase two, agreeing to the outcome.

It was initially applauded by Irish Deputy Prime Minister Simon Coveney: "Deal Confirmed! Ireland supports Brexit negotiations moving to Phase 2 now that we have secured assurances for all on the island of Ireland - fully protecting GFA (Good Friday Agreement), peace process, all-Island economy and ensuring that there can be NO HARD BORDER on the Island of Ireland post Brexit," Coveney said on Twitter.

JUST WATCHED May: 'Ideology' must not impede security deal Replay More Videos ... MUST WATCH May: 'Ideology' must not impede security deal 02:37

But within minutes of Dublin's support came rejection in Belfast from the DUP. "Upon receipt of the draft text on Monday, the Democratic Unionist Party indicated to the Prime Minister that we could not support it as a basis for moving forward."

Hours later, following talks with the DUP, May sent a letter of commitments to Northern Ireland to address the party's concerns.

All of that back and forth made the EU nervous about what May's words actually meant for Brexit -- and more importantly her ability to deliver on them. The EU and UK eventually agreed to move onto the second phase of talks, but few were satisfied that any conclusion over the border had been reached, and many remain skeptical that it can be resolved.

Another stumbling block in all of this has been the collapse of power-sharing talks that would see the Northern Ireland Assembly back on its feet, with the DUP and its archrivals Sinn Fein unable to reach any kind of agreement.

May went to Belfast last week expecting to seal some kind of deal but left empty-handed, with most people blaming the DUP's refusal to accept a deal that would give the Irish language in Northern Ireland the same legal status Welsh has in Wales and Gaelic does in Scotland.

Under normal circumstances, the DUP's hand would not be so strong. But May's members of Parliament know that those 10 DUP votes mean everything in Westminster, where the Conservatives are trying to push through Brexit legislation.

It is perhaps for this reason that some hardline Brexiteers have come to the DUP's aid, pilling pressure on Sinn Fein by accusing it of being behind the failure of the power-sharing talks last week. Worse, some have said that the collapse of those talks proves that the Good Friday Agreement has failed.

Owen Paterson, a Conservative lawmaker and former secretary of state for Northern Ireland, tweeted an article with the headline, "The collapse of power-sharing in Northern Ireland shows the Good Friday Agreement has outlived its use," while Daniel Hannan, a Conservative member of the European Parliament, wrote in an article that the "Belfast Agreement is often spoken about in quasi-religious terms -- literally, for it is more widely known as the Good Friday Agreement. But its flaws have become clearer over time."

David Davis, Britain's secretary of state for exiting the EU, said earlier this week he wasn't aware of the issue when asked about the Good Friday Agreement being "talked down."

By the end of the week, tensions over restarting Northern Ireland's power-sharing Assembly remained elevated, with the DUP contesting reports Sinn Fein was negotiating separately with May. "I'll be raising this matter with the secretary of state," DUP leader Arlene Foster said.

Unlike the DUP's desire for the status quo, Sinn Fein has always wanted a united Ireland -- which it believes Brexit might hasten.

Although the DUP is the biggest party in Northern Ireland, only 44% of people there voted to leave the EU.

Sinn Fein has been more adept at marketing its arguments, both north and south of the border. It has ensured that the government in Dublin maintains pressure on May to uphold her commitments to the EU as stated in the 15-page agreement in December and that she keep the UK government's commitments to the Good Friday Agreement that gives Dublin a say in the affairs of the north.

These are complex and emotive issues for everyone, but they inevitably will all come out in the wash when the reality of May's preferred vision for Brexit gets closer: the UK leaving the EU Customs Union while maintaining a frictionless border with Ireland.

As things stand, these two desires are incompatible.

How, for example, can a non-British citizen arriving by boat or plane in Dublin be stopped from entering the mainland UK by driving an hour north to Northern Ireland without some border controls if, as the DUP insists, Northern Ireland remains seamlessly linked to the mainland?

The pressure on May is mounting when members of her own party are ready to call Northern Ireland's peace process dead.

She came to office under a cloud of rampant Scottish nationalism that was threatening to end a union that has lasted more than 300 years.

Direct from meeting the Queen, before even setting foot inside 10 Downing Street, she told the country she would not countenance Britain's breakup.

"The full title of my party is the Conservative and Unionist Party, and that word unionist is very important to me. It means we believe in the union -- the precious, precious bond between England, Scotland, Wales and Northern Ireland."

Whether she can keep the union together will depend on her ability to pull her hardliners back from the brink of scuttling the Good Friday Agreement, and convincing the DUP that a marginally looser union might be the only one that can survive Brexit.