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.    However, two sources have warned against “reading” too much goodwill in the coming days for discussions with European Commission President Ursula von der Leyen – the agreement will mean additional goodwill, but it also paves the way for an agreement. The agreement will be particularly important for the border between Northern Ireland and the Republic of Ireland, which is to become the ONLY major land border of the United Kingdom (with the exception of the British territory of Gibraltar). The transition period was introduced to allow agreements between the EU and the UK without major interruptions. 🇪🇺🇬🇧 am pleased to announce that @michaelgove and I have reached an agreement in principle on all issues related to the implementation #WithdrawalAgreement through hard work. This ensures that from 1 January, including the Ireland/NI Protocol 👉 t.co/RaWNEVbxrt pic.twitter.com/1OCjapNd3F Since about 2005, the border has been considered invisible, with little or no physical infrastructure, as barriers and checkpoints have been removed as a result of procedures introduced by the Good Friday Agreement (or “Belfast Agreement” signed in 1998).    This agreement has the status of both an international treaty between the United Kingdom and the Republic of Ireland (the Anglo-Irish Agreement) and an agreement between the parties in Northern Ireland (multi-party agreement). 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. The Irish government supported this proposal.  It was strongly rejected by the Democratic Unionist Party as a weakening of Northern Ireland`s place in the UK and is seen as the main reason why Theresa May`s withdrawal agreement was never approved by the British Parliament.
 The British government had rejected the original proposal.