Struggling to win parliamentary support for her Brexit plans – the negotiations that will define Britain’s withdrawal from the European Union in March 2019 – Prime Minister Theresa May is facing her darkest hour.

The central issue this week is that May is trying to win a broad-outline deal with the EU in relation to its customs border with Northern Ireland. Part of the U.K., Northern Ireland also borders the EU member state, Ireland, so it either requires a physical border (think Mexico) that controls trade access with the Ireland, or a deal that allows EU trading rules to be applied in Northern Ireland. But Theresa May and the EU have unable to reach terms on what such a deal might look like. Making matters worse, as she addressed Parliament on Monday, May found herself under fire from all sides as to what she is willing to accept with the EU and how long any transition arrangement might last.