After Brexit, the border between Northern Ireland (part of the UK) and the Republic of Ireland (part of the EU) became the only UK–EU land border. There was concern over this becoming a "hard border" with fewer, controlled crossing points and customs checks. This could have compromised the Good Friday Agreement that ended the Northern Ireland conflict, and lead to violence. All involved parties agreed a hard border should be avoided. To forestall this, the EU proposed a "backstop agreement" 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 supported by the Irish government and many Irish nationalists, but opposed by the Democratic Unionist Party as weakening Northern Ireland's place within the United Kingdom and is regarded as the main reason why Theresa May's withdrawal agreement was never approved by the British Parliament. The UK Parliament had already rejected an earlier proposal.