No rest for Ireland as trouble brews at the border

History never leaves Ireland alone.

In the late 19th and early 20th centuries the lethal alliance of the Conservative Party in Britain and the Unionist Party, which represented the Protestant minority in Ireland, made it impossible for the British parliament to pass a Home Rule Bill for Ireland.

A Home Rule Bill might have let the two countries take their distance peacefully and gradually, while retaining close links – or maybe not – but it was worth a try.

Instead came the Easter Rebellion of 1916, the Irish War of Independence, the partition of the island between the independent Republic and Northern Ireland (part of the UK), the Irish Civil War, and three decades of terrorist war in Northern Ireland that only ended 20 years ago.

Well, the Conservatives and the Unionists are back in coalition now, and another war is brewing on the border between Northern Ireland and the Republic. At the moment it’s practically an invisible frontier, with no border posts or customs checks, because both the United Kingdom and the Irish Republic belong to the European Union. Brexit, however, will put an end to that, and probably to peace as well.

In principle, Britain flouncing out of the EU shouldn’t hurt anybody except the British themselves, but the UK’s Irish border is a nightmare.

Prime Minister Theresa May has sworn a mighty oath that the United Kingdom will leave both the ‘‘single market’’ and the customs union, but that will turn this ‘‘soft’’ frontier into a ‘‘hard’’ EU border with a non-EU country: border guards, customs checks, passports, queues and all the rest.

What made the Good Friday peace agreement of 1998 possible was the promise that the border between the two Irelands would practically disappear, which allowed the Catholic nationalists of the Irish Republican Army (IRA) to believe that their war had not been just a futile struggle that killed 3000 people.

They could dream that with all the coming and going across an open border, the two parts of Ireland would grow closer and eventually reunite.

Recreate a hard border and they will feel cheated. Not all the militants of the IRA will pick up their guns again, but some almost certainly will. It was very hard to stop the first time, and there is no particular reason why a renewed war couldn’t last another thirty years and kill thousands more.

Presumably Theresa May does not want to see this, and the EU recently offered her a way out. If you must go, they said, then leave the inner Irish border open and put your customs and immigration controls between Northern Ireland and the rest of the UK (which are conveniently separated by the Irish Sea).

She must reject that offer because she lost her parliamentary majority in the election she needlessly called last June, and remains in power only thanks to the votes of the Democratic Unionist Party – i.e., the hard-line Protestants of Northern Ireland.

And the DUP, always terrified that Britain will abandon them, simply will not allow any kind of border, however soft, to be put between Northern Ireland and the rest of the UK.

May cannot defy the DUP on this or her government will fall – and the Conservatives would probably lose the subsequent

Not all the militants of the IRA will pick up their guns again, but some almost certainly will.

election, putting her nemesis, the dreaded Labour leader Jeremy Corbyn, in power.

However, if May insists on leaving the EU customs union, there will have to be a ‘hard’ border.

If there is, says Ireland’s Prime Minister Leo Varadkar, he will veto any negotiations between the EU and the UK on a free trade deal after it leaves the Union.

May is finally cornered, and the United Kingdom may end up crashing out of the EU with no deal at all. The UK can then spend the next decade trying to renegotiate on less favourable terms the 59 trade deals it now enjoys with other countries as a member of the EU, and, more likely than not, dealing with a renewed IRA insurgency in Northern Ireland.

Or May could aim for a deal that keeps the UK in the customs union. Then the border would remain open, and there would be no Irish veto, and a reasonable deal on post-Brexit trade would be possible. But that would split the Conservative Party, and avoiding that is far more important to her than all these other issues.