When the United Kingdom voted to leave the European Union in June 2016, it was largely based on a desire to take back control of its borders.

While the UK was a member of the EU, trade and people moved relatively freely through the UK’s “soft” borders based on EU rules. Now, Brexit supporters argue, the UK can decide who and what gets into the UK by “hardening” the border.

This is a fairly complicated process for the UK’s maritime borders. And it has serious consequences for its only overland border — the one between Northern Ireland and Ireland.

The question of how to regulate the Irish border has caused Brexit-EU negotiations to grind to a halt. That’s because the border wasn’t just open due to EU rules. It has been open for 20 years as a compromise that ended decades of violence in Ireland and as a symbol of identity in the country. Closing this border could undermine one of Europe’s greatest success stories.

Watch the video above to see why the Irish border is so important and why it’s such a difficult issue to solve, and subscribe to our YouTube channel to view all of our latest videos.