London - Northern Ireland's Democratic Unionist Party, which is threatening to bring down the British government over Brexit compromises, is a party with hardline views whose tough negotiating tactics were forged in war.
The DUP's 10 members of the House of Commons have propped up Prime Minister Theresa May's government since the June 2017 election, in which her Conservatives lost their majority and were forced to turn to the fellow rightwing party for help.
The agreement came at the price of a promise of £1.0 billion (1.1 billion euros, $1.3 billion) in extra funding for Northern Ireland and the party has done little to risk undermining the government until now.
Its importance in Westminster and the pivotal role of the Irish border issue in negotiations with Brussels has put the party, known for its harsh rhetoric and steely determination, at the sharp end of Brexit.
DUP Brexit spokesman Sammy Wilson on Thursday warned that any checks on goods transiting between Northern Ireland and the rest of Britain, of the type outlined by EU negotiator Michel Barnier on Wednesday, would be "unacceptable".
Accepting this "would have implications not just for Brexit legislation... but also for the budget, welfare reform and other domestic legislation".
- Hardline policies -
The DUP campaigned in favour of Brexit and has supported the government in a series of crucial votes, but is also faced with public concern in Northern Ireland about the possible reimposition of physical checks on the border with the Republic of Ireland.
Both sides have warned that a so-called hard border could threaten the 1998 Good Friday Agreement that brought an end to decades of civil strife between Protestants and Catholics in Northern Ireland.
DUP leader Arlene Foster came under fire last week after suggesting that the peace deal could be altered to shape Northern Ireland's post-Brexit relationship with the EU.
Options for the Irish border
"There has been a lot of misinterpretation, holding it up as a sacrosanct piece of legislation," she said.
The self-styled "Christian fundamentalist" party has softened its fiery anti-Catholicism since it was founded by Protestant evangelical minister Ian Paisley in 1971.
But the party that in 1977 launched the "Save Ulster (Northern Ireland) from Sodomy" campaign still holds hardline religious views, particularly on social issues such as abortion.
Its influence has prompted warnings that a disrupted balance of power in Belfast could harm the delicate peace process.
Irish republicans Sinn Fein have blamed the Westminster power deal for the failure to restart a power-sharing executive with the DUP in Northern Ireland.
That arrangement collapsed in January 2017 because of a row over Foster's handling of a botched renewable heating funding scheme.
The government agreement has also sparked protests in mainland Britain over the DUP's opposition to gay marriage and abortion, as well as many senior members' support for teaching creationism, and a history of links to paramilitaries who fought Catholic nationalists during the Troubles.
Post-Brexit trade options for the UK
Jon Tonge, a professor at Liverpool University who has written extensively about the DUP, said the party has become less dogmatic in recent years, but cannot be described as pluralist.
In his 2014 book "The DUP: From Protest To Power", Tonge found that 54 percent of party supporters "would mind a lot" if someone from their family married a person of another religion, and 58.4 percent would not want their child to go to a non-Protestant school.
It was a surprise to many political commentators in 2005 when the party agreed to enter a power-sharing arrangement with Sinn Fein, once the political mouthpiece of the Irish Republican Army, which fought an armed campaign for Irish unity.
Although the Belfast assembly appeared to operate with reasonable cordiality for much of a decade, months of talks between the DUP and Sinn Fein have failed to re-establish a Northern Irish executive.
The province's finances are currently run by London.