The backstop would not take effect until after the transition period had ended, before all the details of the new relationship were developed. But what is the backstop and why was it such a sensitive point? The Irish backstop was a protocol in the (un ratified) Brexit withdrawal agreement that would have kept the UK (generally) in the customs union of the European Union and Northern Ireland (in particular) on certain aspects of the European internal market until a solution was found to avoid a hard border. This should not compromise the Good Friday agreement and preserve the integrity of the European internal market. This would only have come into effect if there were no other solutions before the end of the (agreed) transition period. Brussels rejected Johnson`s move to eliminate the backstop, making a no-deal Brexit a virtual certainty on 31 October. Given the EU`s interest in maintaining the internal market and the UK`s obligations as a member of the World Trade Organisation, the prospects for a hard border within Ireland seem inevitable in this case. A division of the island would thus jeopardize the provisions of the Good Friday Agreement reached in 1998 and raise the spectre of civil violence. This could be avoided by conducting a referendum in which the citizens of Northern Ireland would vote on whether they want to separate from the UK and be part of a United Kingdom, whether they want to remain members of the EU or to part with it. US Senator George Mitchell, who led the belfast agreement negotiations, said he believed the creation of a border control system between the Republic of Ireland and Northern Ireland could jeopardise the deal.  Surveys published on 18 February 2019 by Irish Senator Mark Daly and two UNESCO Presidents indicated that the reintroduction of a hard border would lead to the return of violence.     The Irish government and the northern Irish nationalists (favourable to a united Ireland) supported the protocol, while the Unionists (who preferred the United Kingdom) opposed it.
In early 2019, the Westminster Parliament voted three times against ratifying the withdrawal agreement, rejecting the backstop. This protocol was strongly rejected by the Democratic Unionist Party, which saw it as a weakening of Northern Ireland`s place in the United Kingdom and is seen by a number of commentators as the main reason why the withdrawal agreement was not ratified by the United Kingdom Parliament.    Since 2018, the DUP has stated that the anti-Northern Ireland ruling must be withdrawn from the Brexit withdrawal agreement if it were to continue to support the Conservative government in the House of Commons although the party has stated that it is open to limiting backstops over time.  This is where the controversial backstop comes into play.