W ithin the EU ’s customs union there are no internal tariffs on goods transported between member countries, and a common tariff on goods entering from outside. What’s more, within its single market there are common standards on matters such as product design and food safety. For this reason there are no customs or regulatory checks at international borders within the EU .

Britain is keen that, after Brexit, it should have an independent trade policy, signing trade deals with other countries and setting its own tariffs. It also wants to take back control of the many rules that are set within the single market. If it does this, checks will have to be carried out on goods being traded between Britain and the EU , to collect tariffs and make sure that products meet standards on either side.

Yet both Britain and the EU have agreed that there must be no new checks at the Irish border. This is partly to maintain the free flow of goods and people, without which the island economy could be disrupted. But the deeper reason is political.

Whereas most members of Northern Ireland’s Protestant majority want the province to remain part of the United Kingdom, most of its large Catholic minority want it to unite with the Republic of Ireland. This was the matter at the heart of three decades of sectarian violence known as the Troubles. The dismantling of the hard border in the 1990s helped to lay the ground for the Good Friday peace agreement of 1998, which enshrined Northern Irish people’s right “to identify themselves and be accepted as Irish or British, or both”. The return of checkpoints or other border infrastructure would risk undermining that principle.

Britain has suggested that technology might be used to carry out customs and regulatory checks without the need for new physical infrastructure. The EU is unconvinced. So the two sides have come up with what they call a “backstop” agreement. Under this, the entire UK would remain in a customs union and in close regulatory alignment with the EU until some way is found to carry out customs and regulatory checks without building new infrastructure at the border. This would keep the border open. But it would prevent Britain from doing trade deals or setting many of its own market rules, until the promised technological solution was found.

Britain’s government has signed up to this plan, as part of its Brexit deal with the EU . But for the deal to take effect, it must be signed off by Britain’s Parliament–and more than 100 Conservative MP s consider the backstop unacceptable. They say, rightly, that the conditions for lifting the backstop might never be met, and that Britain would thus remain in permanent customs and regulatory alignment with the EU . They describe this as “vassalage”, since Britain would have to obey rules it had no say in setting. The trouble is that they haven’t come up with a viable alternative.

The picture is further complicated by the fact that Theresa May’s Conservative government depends on an alliance with the Northern Irish Democratic Unionist Party. The DUP backed Brexit. But it particularly dislikes the backstop arrangement, as it would involve Northern Ireland being in closer regulatory alignment with the EU than the rest of the UK . As unionists, they are appalled by the idea of regulatory divergence between Northern Ireland and Britain. If Mrs May somehow manages to get Parliament to agree to her deal, the DUP has indicated that it would vote with the Labour opposition to bring down her government.

For the past 20 years Britain’s political class has all but ignored Northern Ireland. Yet two decades after the end of the Troubles, the Irish question is back–and it is more likely than any other matter to derail Brexit.