In 2001, civil war broke out at Cooper Wilkinson, a family solicitors’ practice in Enniskillen, Northern Ireland. The local Ulster Unionist MP, Ken Maginnis, was retiring and a succession battle was under way. His election agent of 25 years, James Cooper, had expected to inherit the seat but faced a challenge from one of his employees: Arlene Foster, a 30-year-old solicitor.

Unlike Cooper, Foster had opposed the 1998 Good Friday Agreement, objecting to the mandatory release of republican terrorists. The agreement had brought an uneasy peace to Northern Ireland but had split the largest party of its unionist majority, David Trimble’s UUP.

A member of a hard-line faction of Queen’s University Belfast-educated lawyers known as the “baby barristers”, Foster was one of the party’s leading dissenters and argued that her boss would lose the seat to Sinn Féin if selected. Cooper was chosen nonetheless, but Foster was vindicated when he lost. Within three years, Foster had quit the UUP for Ian Paisley’s Democratic Unionist Party, which she now leads.

Today, the 48-year-old’s refusal to compromise is causing headaches for her latest senior partner: Theresa May. The DUP’s ten MPs, on whom the Prime Minister relies for a parliamentary majority, are refusing to support her Brexit deal.

Foster, a member of the suspended Stormont assembly, is not among them, but has arguably become the most prominent Northern Irish politician since the Troubles. Her reputation for public truculence has won her a nickname that belies her private warmth and charm: Snarlene.

Yet despite one-dimensional coverage of the DUP in the English media, Foster does not share Paisley’s antediluvian outlook – she is a member of the Church of Ireland, not the hard-line Free Presbyterians (the leader she still affectionately knows as "Doc" told her she was joining a political party, not a church) – and rose to political prominence in ministerial office, not on the streets.

Born Arlene Kelly in 1970, Foster grew up as part of the Protestant minority in rural Co Fermanagh, on the very border whose fate will define her political career. One night in 1978, her father, John Kelly, a farmer and part-time policeman, was shot in the head while tending to his livestock.

Then just eight years old, Arlene witnessed him crawl bloodied into their kitchen (the suspected gunman, Séamus McElwaine, was later eulogised by Sinn Féin’s Martin McGuinness). He survived and lived for another 32 years, but his family were forced to abandon their farm and move away from the border to the town of Lisnaskea.

From there, the young Arlene Kelly travelled by bus to grammar school in Enniskillen, a journey that was interrupted by an IRA bomb one morning in 1988 (the driver was a part-time soldier). The ordeal – which claimed no lives – would win Foster an unlikely distinction: she is perhaps the only British politician whom Jeremy Paxman has subjected to a sympathetic line of questioning.

Elected to Stormont in 2003, Foster was talent-spotted by Peter Robinson, Paisley’s wily deputy, after her defection to the DUP. She succeeded him as the first female leader of the DUP and of Northern Ireland in 2016, serving as first minister alongside McGuinness.

Much has changed since: McGuinness resigned in January 2017, criticising Foster’s attitude to nationalists and minority groups, as well as her handling of the Renewable Heat Incentive scandal – a subsidised energy scheme to which she failed to introduce cost controls while Northern Ireland’s enterprise minister. It is thought to have cost the taxpayer £500m, and a subsequent inquiry has revealed the deep dysfunctionality of devolved government (Foster admitted she did not read the relevant legislation). The power-sharing executive collapsed: in the ensuing election the DUP lost ten seats and unionism its Stormont majority.

McGuinness died last March and Foster has failed to strike up a working relationship with his successor, Michelle O’Neill. Devolved government remains suspended, but any surviving doubts about Foster’s leadership were dispelled when the DUP won ten seats – and unprecedented influence – in the 2017 UK general election. (Though Foster can count of the support of Nigel Dodds, the party's Westminster leader and eminence grise, there are persisent sotto voce briefings about her future that could yet burst into the open when the RHI inquiry reports its findings.)

Shorn of the obligations of government and led by more doctrinaire MPs in Westminster, the party’s rhetoric on Brexit hardened: where once Foster had been willing to request special solutions for Northern Ireland alongside McGuinness, now she vigorously opposes them.

The no-deal breach with Brussels that Foster’s MPs might yet facilitate could destabilise Northern Ireland’s place in the United Kingdom. And the ultimate price could be Irish unity within Foster’s lifetime.

Is she prepared to pay it? “If it were to happen, I’m not sure that I would be able to continue to live here… I would probably have to move,” she told a BBC documentary hosted by an old Queen’s classmate, the comedian Patrick Kielty, in April. Unlikely as it would have seemed at the outset of her political career, that may yet become a self-fulfilling prophecy.