Northern Ireland is back on the front pages, from “where it has been happily absent for 20 years”, as Jonathan Powell, Tony Blair’s former chief of staff, put it in The Guardian.

The collapse of the devolved power-sharing government at Stormont, the Tories’ confidence-and supply-arrangement with the Democratic Unionist Party (DUP) and Brexit have resurrected a whole host of political spectres – not least the Good Friday Agreement.

What is the Good Friday Agreement?

Also known as the Belfast Agreement, this complex accord laid out how Northern Ireland would be governed. It was signed on 10 April 1998 and sought to put an end to 30 years of the Troubles. Largely, it succeeded but now faces fresh challenges that will test it to the limit.

What were the terms?

The agreement prepared the ground for a devolved power-sharing model in a new 108-member Northern Irish assembly. Under its rules, no one party would be able to control the assembly, in an attempt to solve the longstanding religious tensions between Protestants, who make up the majority of the population of Northern Ireland, and Catholics.

It also created institutions linking Northern Ireland with the Republic of Ireland and Westminster, as well as laid out proposals for decommissioning paramilitary weapons and releasing paramilitary prisoners.

The deal was put to the vote across Ireland – and both Blair, who was then prime minister, and the Irish government knew it needed to earn significant support to be credible.

Ian Paisley’s DUP was the only major party to oppose the deal, fearing it would threaten Northern Ireland’s union with Great Britain.

Despite this, the agreement secured the support of 71% of voters in Northern Ireland and 94% in the Republic.

First elections to the new assembly were held in June 1998, with the nationalist Social Democratic and Labour taking the largest share of the vote, followed by the Ulster Unionist Party, the DUP and the nationalist Sinn Fein.

So that was that?

Not quite. The new executive model proved problematic and distrust between the parties led to the assembly being suspended several times. From 2002 until 8 May 2007, when the St Andrews Agreement was signed, Northern Ireland was once again directly ruled from Westminster.

However, weeks before he stepped down, Blair managed to win over Paisley, despite previously saying the idea of the DUP man agreeing to a deal was “pie in the sky”, the BBC reports.

The new accord made ministers more accountable to the executive and the assembly, while the first minister and deputy first minister would now be nominated by their parties rather than elected.

And that solved the problems?

Wrong again. Power-sharing collapsed again at the beginning of 2017, when deputy first minister Martin McGuinness of Sinn Fein resigned, accusing the DUP of “arrogance” in their handling of a botched energy scheme, The Independent says. Since Sinn Fein refused to table another deputy and the DUP’s Arlene Foster lost her role as first minister. She had previously rejected calls to resign.

The assembly collapsed when talks broke down in March, with both Sinn Fein and the DUP blaming each other. McGuinness himself died later that month.

Since then the DUP and Sinn Fein have been unable to come to an agreement on power-sharing. The latest deadline set by the government in Westminster passed in February, with both sides blaming each other for the impasse.

The main disagreement appears to be over Sinn Fein’s demands for new legislation to safeguard and promote the Irish language, something the DUP has categorically ruled out.

Despite the best efforts of the British government, former US president Bill Clinton, who was instrumental in brokering the original Good Friday Agreement, and even the offer of help from Donald Trump, both sides remain at loggerheads.

In the absence of a devolved executive, the British government this month moved to impose a budget directly on Northern Ireland; in what former Labour Northern Ireland secretary Peter Hain has said amounts to “direct rule in all but name”.

How does the Tory-DUP alliance affect the situation?

The UK government has always sought to remain impartial in negotiations, supporting neither the unionists nor the nationalists.

However, with the DUP propping up Theresa May’s government, many nationalists claim this rings hollow.

The DUP has been keen to offer reassurances its demands largely concern social policy and that sectarian issues will be kept away from the agenda, says the Guardian. Still, it has sought assurances that there will be no referendum or border poll on Irish unity as part of the deal with the Tories.

Following the collapse of power-sharing talks, and the effective reimposition of direct rule, Reuters says Irish nationalists “fear that that their unionist rivals may effectively govern the province through their influence of May”.

Speaking after last June’s general election, the then Sinn Fein leader Gerry Adams warned a Tory-DUP alliance would lead to a “coalition of chaos”, and could put the Good Friday Agreement at risk.

In his piece in the Guardian, Powell, who was the chief British negotiator during the Good Friday Agreement talks, echoed this views, calling the Tory-DUP alliance a “a terrible mistake with lasting consequences” that “would risk undermining 20 years of hard work”.

Will it have any impact on Brexit?

All this has to be seen in the wider context of Brexit.

Northern Ireland voted Remain by 56% to 44%, but the decision taken in the rest of the UK has raised the spectre of a post-Brexit “hard border” between north and south and revived sectarian tensions.

Britain, Ireland and the EU have repeatedly stated their desire to avoid a hard border, understood to mean any kind of physical border or visible customs checks. But many claim a frictionless border is incompatible with Theresa May’s promise to leave the EU customs union.

Sinn Fein and the unionists are touting a united Ireland as an alternative means to remain in the EU, something the DUP vehemently opposes, while local officials fear a return to “us versus them” politics of the past, the Guardian reports.

George Mitchell, who as US Special Envoy for Northern Ireland chaired the talks that led to the historic agreement 20 years ago, has warned that a perfect storm of direct rule and a hard border could lead to “serious trouble” and a return to the violence of the 1970s and 1980s.

Speaking on the BBC’s Today programme ahead of the 20th anniversary, he said a hardening of the border could mean “stereotyping resumes, demonisation resumes and people turn inward”, while a failure to restore power-sharing could lead “regressive forces” to reignite their campaign of violence.

Northern Irish leaders should recognise this as a “dangerous signal … and act before there is a regression”, he added.