UK Prime Minister Theresa May has announced that members of Parliament (MPs) are set to vote for a second time on the Brexit withdrawal agreement on March 12. The agreement suffered a historic loss on January 15 when MPs rejected the deal by 432 votes to 202.

May has returned to Brussels for further negotiations on the withdrawal agreement with the European Union in hopes of securing the kind of modifications that the British lawmakers say they need to pass it. So far, EU officials have indicated that they don’t intend to re-open the previously agreed-upon withdrawal agreement. If MPs reject her deal again on March 12, they are expected to vote on whether or not to leave the EU without a deal in place, and whether or not the March 29 exit date can be pushed back. Although there is a wide range of complaints across Parliament, there are a few select sticking points.

The main source of opposition is the border between Ireland and Northern Ireland, or the backstop. Historically, a hard customs border dividing the two nations has caused violence and unrest. Both the EU and the UK wish to avoid a hard border between the two. Consequently, the first draft of the withdrawal agreement included the backstop, which is designed to act as a temporarily fluid border between EU-member Ireland and Northern Ireland, which is part of the UK. However, MPs are concerned that the backstop would either become a permanent measure or a source of division in the UK if Northern Ireland enjoyed trade advantages that the rest of the nation did not.

Although it is written in law, according to Article 50 of the Treaty of Lisbon, that the UK is set to leave the EU on March 29, nothing is certain at this point. The March 29 deadline could be extended, though Prime Minister May said a delay wouldn’t make securing a deal “any easier.” The European Court of Justice has stated that the UK retains the option to cancel Brexit altogether. This would mean that the UK would hold a second referendum in which the public would vote again whether or not to leave the EU, though this scenario remains unlikely.

Struggling to keep up with the Brexit timeline? If you would like to stay up-to-date, you can read our comprehensive timeline of Brexit key dates and players, visit BBC’s guide to the UK leaving the EU, or read Reuter’s coverage of the Brexit vote delay.