Illustration: Aïda Amer/Axios

In the 1,160 days since the Brexit referendum, Northern Ireland — a complex, but oft-forgotten country the size of Connecticut — has emerged as the ultimate sticking point in the worst crisis the U.K. has faced since World War II.

Why it matters: Brexit threatens to unsettle the dual identity dynamic on which peace in Northern Ireland hinges. The struggle to maintain that balance has ended the political career of Theresa May, catapulted Boris Johnson into Downing Street, and could result in a cliff-edge Brexit on Oct. 31 with potentially disastrous consequences — barring a miraculous last-minute deal.

The big picture: Northern Ireland is home to the U.K.’s only land border with an EU member state. It’s there — where a physical border with checks and infrastructure has not existed for 2 decades — that complex issues of identity, sectarian violence and trade have coalesced into Brexit’s most intractable puzzle.

Background: Between 1968 and 1998, more than 3,000 people were killed and nearly 50,000 injured in sporadic bouts of politically motivated violence known as "The Troubles."

  • The low-intensity armed conflict saw nationalist paramilitaries — most famously the IRA — carry out guerrilla campaigns against state security forces in the name of ending British rule and reuniting Ireland.
  • Loyalists, who sought to remain part of the U.K., retaliated with attacks and bombing campaigns of their own, including against the minority Catholic community.
  • The conflict was formally ended in 1998 by the Good Friday Agreement, which established Northern Ireland’s current system of government and created some shared institutions with Ireland. Perhaps most importantly, it demilitarized and essentially abolished all visible signs of the Irish border.

Between the lines: Some people in Northern Ireland feel Irish. Others feel British. After the Good Friday Agreement, the lack of a hard border — since both Ireland and the U.K. are in the EU — made it possible for people to identify with either.

  • George Hamilton, the chief constable of Northern Ireland’s police service, tells the BBC that any new border infrastructure installed after Brexit would be attacked by Irish nationalists: "If you put up significant physical infrastructure at a border, which is the subject of contention politically, you are re-emphasizing the context and the causes of the conflict."

Flash forward: Theresa May was unable to pass her Brexit deal through Parliament because of opposition to the so-called "backstop," an insurance policy that would keep the U.K. aligned with the EU’s customs rules in case the two sides couldn’t figure out another way to avoid a hard Irish border by 2022.

  • New Prime Minister Boris Johnson has demanded that the backstop be removed, arguing that it would trap the U.K. inside the EU’s orbit indefinitely. The EU has refused, stressing that it’s simply a backup policy to ensure the Good Friday Agreement is preserved.
  • Johnson has claimed that "under no circumstances" would the U.K. put up border checks in Northern Ireland, but he has not proposed any viable alternatives to enforcing customs rules after Brexit. Leaked documents from his own government suggest that an open border would be "unsustainable" in the event of a no-deal Brexit.

What to watch: Parliament returns from recess next week, where its first order of business will be staging a cross-party rebellion to stop a no-deal Brexit. But lawmakers' power was severely hampered by Boris Johnson's dramatic decision on Wednesday to suspend Parliament from Sept. 11 to Oct. 14, giving them limited time to get their act together.

Go deeper: More on what Boris Johnson's suspension of Parliament means for Brexit

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