Few places are seeing a more passionate debate about Brexit than Northern Ireland.
It voted to remain in the EU by 56% to 44%, and feelings are running high about the consequences of Brexit because this is the only part of the UK that shares a land border with the EU.
The future of the Northern Irish border is therefore featuring prominently in the negotiations between Brexit Secretary David Davis and Michel Barnier, his opposite number from the European Commission.
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Leave aside the security concerns – which, of course, are huge.
No one wants to see the re-imposition of a hard border of the kind that existed prior to the peace process, and yet the logic of the UK regaining control of its borders would point to exactly that outcome – unless the risk of migrants entering Britain from other parts of the EU, via Ireland, is going to be ignored.
The trade issue is equally vexed.
Mr Davis published a position paper earlier this month which suggested trade between Northern Ireland and the Republic would remain roughly as it is today. Mr Barnier said what he saw in the paper “worried” him.
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Ireland is worried too and for the same reasons.
No border controls with the Irish Republic would mean – assuming Britain succeeds in cutting free trade deals with other parts of the world after leaving the EU – the possibility of goods being imported from outside the EU into Belfast and then being sent down south to be exported on a tariff-free basis to other parts of the EU.
That could potentially undercut the competitiveness of EU-based producers of those goods.
And now other specific concerns are coming to light about the kind of trade deals the UK will be seeking to cut post-Brexit.
Boeing, the American aircraft making giant, is currently pursuing a case against Bombardier, the Canadian aircraft maker, which it says is selling its C Series narrow-bodied aircraft more cheaply than it should be.
The US government is investigating whether this amounts to ‘dumping’ and is also looking into whether Bombardier has received illegal state aid, not just in Canada, but also from the UK.
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Bombardier is one of the most important employers in Belfast and the company has received a loan from the UK Government towards the costs of developing the C Series.
The Belfast factory, previously known as Shorts, employs 4,500 people. The majority of them are Protestant and vote for the Democratic Unionist Party (DUP), which is providing support in Westminster to Theresa May’s Government.
The future of the plant is therefore something taken very seriously by the DUP. Its leader, Arlene Foster, has said it is worth some £183m to the Northern Irish economy in wages alone – and has assumed critical importance in recent days to the extent that it came up in conversation during a phone call last week between Mrs May and President Trump.
Boeing would like to see Bombardier hit with tariffs if it can be found to have been selling at an artificially low price.
The future of the plant, bought by Bombardier in 1989, is likely to be among the main topics of conversation when Mrs May meets Canadian PM Justin Trudeau later on Monday – although another priority will be ensuring that a bilateral trade deal, replicating the one the EU has just agreed with Canada, can be struck between Britain and Canada post-Brexit.
The spat highlights neatly some of the difficulties Britain will have as it seeks to forge new trading relationships with the rest of the world after leaving the EU.
Supporting British jobs by striking free trade deals may not always be compatible with good relations with other countries whose businesses also support British jobs. That is no different from the current arrangement Britain has within the EU.
However, with the aerospace industry currently exempt from EU tariffs, there is a danger some EU producers of particular parts might push, post-Brexit, for tariffs to be imposed legally on raw materials used in the manufacture of aerospace components to reduce the competitiveness of British rivals.
It’s easy to see how issues like this might hamper Britain’s ability to strike bilateral trade deals in future.