London (CNN) One of the big fears in the Brexit debate is that Britain's departure from the EU will mean the reintroduction of border posts on the frontier between Northern Ireland, which remains part of the UK, and the Republic of Ireland, an EU member.

Border infrastructure was often targeted by Irish nationalist paramilitaries during the "Troubles" -- the 40-year sectarian conflict in which more than 3,500 people died.

The issue of the border status is already reigniting passions in Northern Ireland.

UK Prime Minister Theresa May's Brexit deal avoids the reintroduction of a so-called "hard" Irish border, because of the built-in transition period that keeps the UK in a customs union with the EU.

The problems come if there's no agreement on what to do after the transition period ends in 2020. Enter the Irish "backstop," an insurance policy designed to avoid a hard border if no other solution to police the border are found by that time.

This has infuriated hard-line Brexiteers, worried the UK will never "properly" leave the bloc. They want to be free of the customs union, in order to forge international trade deals that would require the UK to be free of EU regulations on issues like agriculture, fisheries, food standards and the environment.

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