No mainstream British political party is more comfortable with the rhetoric of unionism than the Conservatives. It never takes long before Tory ministers, confronted with some challenge or other from Irish, Scottish or Welsh nationalists, fall back on the mantra that they represent the Conservative and Unionist party. Theresa May does this all the time. The implication is always the same. The union is axiomatically safe in Tory hands, because they, the Tories, uniquely understand and are uniquely committed to it.

Except, this week, they showed that they don’t and they aren’t. There have been multiple humiliations of government Brexit policy this week. There is a luxurious display of catastrophes to choose from. They range from the jaw-dropping confession by Philip Hammond that the cabinet has never ever, in 18 months since the referendum, held a single discussion about what kind of Brexit it seeks, to the air rushing rudely and loudly out of the whoopee cushion of David Davis’s Trumpian claims to be one of the negotiating masters of the universe after he admitted that he had conducted not a single assessment of the impact of Brexit on any sector of the UK economy.

Yet perhaps the most damning failure of a disastrous week for May is the evidence that the modern Conservative party simply does not understand any of the cultural contours of Britain. It is not a surprise that a lot of Conservatives don’t like abroad, or that they dislike the EU in particular. But the more extreme failure is that they don’t get Northern Ireland – or the Irish Republic either. They tend to forget Scotland. They don’t really think about Wales at all. Gibraltar, whose union jackery the Conservatives will always loudly pledge to defend, never so much as enters their minds, though in Brexit terms it is part of this country too.

And if they don’t get any of these places right, they don’t really get England right either. The desire to leave the EU, it became clear this week, is based not just on a failure to understand Europe but on a profound set of misconceptions about these islands. The two failures are umbilically linked, which is why Brexit is a profound threat not just to relations with Ireland but to the union itself.

Quick Guide Why is the Irish border a stumbling block for Brexit? Show Counties and customs Inside the EU, both Ireland and Northern Ireland are part of the single market and customs union so share the same regulations and standards, allowing a soft or invisible border between the two.

Britain’s exit from the EU – taking Northern Ireland with it – risks a return to a hard or policed border. The only way to avoid this post-Brexit is for regulations on both sides to remain more or less the same in key areas including food, animal welfare, medicines and product safety. The 'backstop' in Theresa May's Withdrawal Agreement was intended to address this - stating that if no future trade agreement could be reached between the EU and the UK, then rules and regulations would stay as they are. This has been rejected by Brexit supporters as a 'trap' to keep the UK in the EU's customs union, which would prevent the UK striking its own independent trade deals. There are an estimated 72m road vehicle crossings a year between Northern Ireland and the Republic of Ireland, and about 14% of those crossings are consignments of goods, some of which may cross the border several times before they reach a consumer. Brexit supporters say this can be managed by doing checks on goods away from the border, but critics say it will be difficult to police this without any physical infrastructure like border posts or cameras, which could raise tensions in the divided communities of Ireland.

Interactive: A typical hour in the life of the Irish border Photograph: Design Pics Inc/Design Pics RF

That there would be a problem in Ireland with the Tories’ approach to Brexit has long been obvious. Some of us had warned about this issue before the vote, and then warned about it again afterwards. Yet May breezed on, her admirers berating the sceptics as remoaners, confident she could have it all: leave the single market and the customs union and yet continue the open border between north and south in Ireland. This week, in the most humbling of circumstances, the border bit back and the sceptics had their best week since the election.

There were signs today that London may have found some more capacious form of words that will enable the Irish Republic, the EU, the DUP and the government to allow the Brexit talks process to get into its second phase at next week’s Brussels summit. At a pinch, the decision on a form of words may be pushed into 2018. But the whole of this week’s crisis has been caused by the collective and institutional failure to take the Irish dimension seriously enough.

How could a London government that is peopled by ministers who are still of an age to remember what Northern Ireland was like in the 1970s, and how radically this changed in the 1990s, forget so quickly? It is incredible that supposedly serious British politicians should simply overlook the trenchancy of DUP attitudes towards the status of Northern Ireland, allow the zero-sum politics of the north to slip their minds or lose track of the Irish Republic’s existential engagement with the peace process. But it has happened. The very fact that the DUP Westminster deal exists at all is proof that political antennae have atrophied. A government whose mind was large enough to think about the Irish dimension properly would have been far more careful.

“You are all so young,” lamented the admirable Lady Hermon during an important debate this week on the EU withdrawal bill in which she, the sole independent unionist voice in the Commons, tried (and failed, alas) to get the government to write the upholding of the Good Friday agreement into the bill. Yet the passing of the generations at Westminster, when only those in their 50s know what Burntollet Bridge, the Bogside, Warrenpoint and all the rest mean, only partially explains why British politicians don’t take Ireland seriously enough. The lamentable absence of a constitutional nationalist voice in the 2017 parliament – for the first time since the first half of the 19th century – is another concern too.

But there is surely a deeper cultural explanation as well. There is a closing of the mind in British society and politics, a lack of awareness, understanding, experience, imagination and contact with those who live outside the political class or who see the history of these islands differently. It is a complicated material and cultural process, not to be trivialised as merely the failings of individual politicians. It also reflects a shared civic failure and decline in basic statecraft. But its results are dire.

Among them are the mental retreat that led both to the referendum result itself and now to the subsequent mishandling of Brexit in its turn, as well as to the widely acknowledged inability of the government to focus effectively on anything except Brexit. May, with her cavalier disregard of the 48%, is very lucky she has Ruth Davidson to at least remind her that these things look different from Scotland. Davidson this week understood what May, in spite of her unionist claims, did not. The Irish draft this week was a gift to Scottish separatism. The logic of the Irish problem can only mean a soft Irish border and regulatory alignment with the EU in the form of continued membership of the customs union.

No British prime minister, especially one who actively promotes herself as a defender of the union as May does, can be excused for paying so little attention to Northern Ireland and to relations with the Irish Republic. May cannot be everywhere all the time, but she has been to the Middle East more often than she has been to Ireland since becoming prime minister, and she spends a lot more time in Brussels than she does in Edinburgh or Cardiff.

Nor does she have heavyweights to fill in for her, either as secretaries of state or at No 10. In the end, this shows in the outcomes. And that is what happened this week, when her signature policy was exposed as delusional, incompetent and unworkable.

• Martin Kettle is a Guardian columnist