LONDON (AP) — The election of Sinn Fein as the biggest party in Northern Ireland’s Assembly is a historic moment — the first time an Irish nationalist party, rather than a British unionist one, has topped the voting.

With all but two of the assembly’s seats filled Saturday, Sinn Fein has won with 27 seats out of 90. The Democratic Unionist Party, which had been the largest for two decades, has 24 seats and the Alliance Party, which defines itself as neither nationalist nor unionist, has 17.

WHY IS THIS A BIG DEAL?

The outcome is hugely symbolic. A party that aims to unite Northern Ireland with the neighboring Republic of Ireland has a mandate to take the reins in a state established a century ago as a Protestant-majority region within the United Kingdom.

It’s a major milestone for a party long linked to the Irish Republican Army, a paramilitary group that used bombs, bullets and violence to try to take Northern Ireland out of U.K. rule during decades of unrest. More than 3,500 people died in 30 years of violence involving Irish republican militants, Protestant Loyalist paramilitaries and the U.K. army and police.

A 1998 peace accord ended large-scale violence and Northern Ireland now has a government that splits power between British unionists and Irish nationalists. The arrangement has often been unstable, but has endured.

WILL SINN FEIN NOW GOVERN NORTHERN IRELAND?

The result gives Sinn Fein the right to hold the post of first minister in Northern Ireland’s power-sharing government, with the DUP taking the deputy first minister role.

But it’s unlikely a government will be set up smoothly soon.

Under Northern Ireland’s delicate power-sharing system, the posts of first minister and deputy first minister have equal status, and both posts must be filled for a government to be formed.

While Sinn Fein is ready to nominate its Northern Ireland leader Michelle O’Neill as first minister, the DUP says it will not follow suit unless there are major changes to post-Brexit border arrangements that it says are undermining Northern Ireland’s place in the U.K.

WHAT DOES BREXIT HAVE TO DO WITH IT?

Britain’s decision in 2016 to leave the European Union and its borderless free-trade zone has complicated Northern Ireland’s position. It is the only part of the U.K. that has a border with an EU nation. Keeping that border open to the free flow of people and goods is a key pillar of the peace process.

So instead, the post-Brexit rules have imposed customs and border checks on some goods entering Northern Ireland from the rest of the U.K. — a border in the Irish Sea, rather than on the island of Ireland.

Unionists say the new checks have created a barrier between Northern Ireland and the rest of the U.K. that undermines their British identity. The largest unionist party, the DUP, is demanding the arrangements, known as the Northern Ireland Protocol, are scrapped.

Britain’s Conservative government says the arrangements cannot work without unionist support, and is pressing the EU to agree to major changes. British Prime Minister Boris Johnson has threatened to unilaterally suspend the rules if the bloc refuses.

But the U.K.-EU negotiations have reached an impasse, with the bloc accusing Johnson of refusing to implement rules he agreed to in a legally binding treaty.

WHAT HAPPENS NEXT?

The Northern Ireland Assembly must meet within eight days so the newly elected legislators can take their seats. Assembly members will then choose a Speaker, followed by the nomination of ministers, starting with the first and deputy first ministers.

If, as seems likely, no executive can be formed because the DUP refuses, ministers from the previous government will stay in power and basic governance can continue — though ministers are barred from making major or controversial decisions.

If there is still no executive after 24 weeks, a new election must be held.

IS IRISH REUNIFICATION LIKELY?

Irish unity did not play a big role in this year’s Northern Ireland election campaign, which was dominated by more immediate worries, especially a cost-of-living crisis driven by the soaring costs of food and fuel.

But it remains Sinn Fein’s goal, and party leader Mary Lou McDonald says a referendum in Northern Ireland could be held within a “five-year framework.”

The 1998 Good Friday peace deal stated that Irish reunification can occur if referendums support it in both Northern Ireland and the republic.

In Northern Ireland, such a vote would have to be called by the British government, “if at any time it appears likely to him that a majority of those voting would express a wish that Northern Ireland should cease to be part of the United Kingdom and form part of a united Ireland.”

There are no set rules for deciding when that threshold has been met.

Complicating the picture is the fact that Northern Ireland’s identity is in flux, with a growing number of people — especially the young — identifying as neither unionist nor nationalist. That is reflected in the strong showing of the centrist Alliance Party. There are growing calls for the power-sharing rules to be changed to reflect the move beyond Northern Ireland’s traditional religious and political divide.

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