The potential impact on the border between Northern Ireland and the Republic of Ireland has been a contentious issue. Since 2005, the border has been essentially invisible. After Brexit, it became the only UK–EU land border (not counting the land borders EU states, Spain and Cyprus, have with British Overseas Territories). All involved parties agreed a hard border should be avoided, as it might compromise the Good Friday Agreement that ended the Northern Ireland conflict. To forestall this, the EU proposed a "backstop agreement" (the Northern Ireland Protocol) that would have kept the UK in the Customs Union and kept Northern Ireland in some aspects of the Single Market also, until a lasting solution was found. The backstop was part of the withdrawal agreement, but was replaced in the revised agreement.