The Irish backstop: Brexit's biggest hurdle

Since Britain voted to leave the EU, the Irish “backstop” has become the primary obstacle in the way of a Brexit deal, say Alex Barker and Arthur Beesley. The plan is the result of intense diplomacy by Dublin, but Theresa May faces implacable opposition to it from Brexiters and Arlene Foster’s DUP. Just how did the issue become so important? Produced by Harry Robertson