A cursory walk down the streets of central Belfast reveals a city in revival. Bustling pedestrian walkways provide access to numerous bars and cafes all thrumming with office workers enjoying a pint on their lunch break. Extravagant graffiti, once viewed as provocative and inflammatory, have largely been appropriated and adopted as part of the city’s charismatic charm. Not far from the center of the city lies the former docks, where a deep local pride in the craftsmanship behind the building of the Titanic continues to linger as developers eye new opportunities along its formerly prestigious waterways. Nestled in the heart of it all lies Stormont, the impressive parliamentary building of Northern Ireland, which is notably devoid of any flags, British or otherwise.

For anyone that remained solely within the center of Belfast, they’d be forgiven for coming away convinced that the city had shed its the scars of the past and had healed its wounds. However, an excursion into the nearby neighborhoods unearths a community still very much divided. No more than five minutes from Stormont are streets where hundreds of British and Ulster flags hang from telephone poles and houses and  where graffiti declaring the protection of local Loyalist paramilitary groups are ever-present. Walk just another couple of minutes and you’re surrounded by Irish flags and monuments to fighters of the Irish independence movement.

In recent weeks, UK Prime Minister Theresa May has held a firm, yet ambiguous, position on the outcome of the UK referendum to leave the EU. She has sworn to respect the outcome of the referendum and has already announced that continued freedom of movement with the EU would be a “red line” in negotiations with Brussels for access to the single market. However, she has also struck down the idea of implementing a points-based immigration system, as seen in Australia, and her administration has steered away from David Davis’ claims that negotiating to remain in the single market would be highly improbable.

Undoubtedly, May’s ambiguity, and at times seemingly conflicting stances, belie the enormous challenges that she faces in securing a negotiated outcome with Brussels that will allow her to respect the referendum results while also preventing economic and political turmoil in the British Isles. While much focus has been on the prospect of Nicola Sturgeon renewing a push for Scottish Independence and the flight of FDI and businesses to the European mainland should Westminster not secure continued passporting rights and access to the single market, May’s ability to extricate the UK from the EU as part of a “hard Brexit” is further complicated by a far more foreboding and politically charged reason: the Northern Ireland conflict.

The Good Friday Agreement stipulates that the final status of Northern Ireland is to be decided by the people of Northern Ireland and the Republic of Ireland and that residents of Northern Ireland should be recognized as citizens of both the UK and Ireland. A Common Travel Area encompasses the British Isles, allowing individuals of the UK and the Republic of Ireland to cross each other’s border without restriction or pause. UK and Irish citizens also maintain the right to reside and retire in each other’s countries and are entitled to the same rights to services as local citizens, rights that precede and extend beyond the rights granted to other EU nationals and that are enshrined in both countries’ immigration laws. Able to share the benefits and nationality of both sides, each individual of Northern Ireland is free to interpret their own identity, a compromise that has suppressed questions of national sovereignty indefinitely.

It is this fragile status quo that presents one of the biggest headaches for Theresa May. So long as both the UK and Ireland are members of the EU, their special relationship is tenable under EU law; however, the prospect of a complete British exit from the EU poses major legal questions about the future of such agreements with Ireland. Post-Brexit, Ireland officially becomes an EU border country, just like Greece and Poland; thus, it is mandated to maintain proper border control and checks on countries not within the EEA or EU. Exemption from each other’s immigration policies means that Ireland and the UK have, in effect, their own separate Freedom of Movement agreement. Should the UK refuse to accept Freedom of Movement between itself and all EU members—as many pro-Leave voters want —this could mean that the UK and Ireland’s arrangement violates EU law.

The realities of a full Brexit and continued Irish EU membership ensure an onerous series of negotiations between Westminster and Brussels that will inevitably give Theresa May pause. While Westminster will undoubtedly find support in Dublin for an exemption to EU freedom of movement and for the continuation of the common travel area based on Northern Ireland’s extraordinary legal status, it still will have twenty-six other governments to convince, many of whom may be wary of the precedent such an arrangement can set. While the EU has rules in place that prevent EU member nation-owned territories from access to the EU (e.g. French Guyana), there is no current arrangement that allows freedom of movement, goods, and services between a single EU-member and a non-EU state.

Exacerbating the situation is the upcoming elections in both France and Germany. While neither is looking to indirectly cause Ireland harm, any effort to provide what constituents might see as even more “exceptional” rights to the UK could be politically dangerous. With the stability of the European Union on the line, the future status of Northern Ireland and the Common Travel Area may provide significant leverage in disabling the UK’s hand when it comes to negotiating over freedom of movement and single market access for British goods and services.

Which is where the troubles rise for May. Northern Ireland is heavily dependent upon EU aid and voted in favor of remaining within the EU in June’s referendum. The prospect of losing that financial support and facing a hard border between itself and the Republic of Ireland may place an unprecedented burden on the citizens of Northern Ireland. For the sizable minority of Northern Irelanders in favor of unifying with the Republic of Ireland, these impositions would be unacceptable and would likely spark instability and renewed calls for unification. Indeed, Northern Irish political leaders have already sent their conditions to May, demanding that the Common Travel Area and access to Ireland be preserved. Additionally, private citizens have already sued Westminster to prevent the UK leaving the EU, arguing on the grounds of a full Brexit being in violation of the UK’s treaty obligations under the Good Friday Agreement.

A Brexit that takes Northern Ireland with it will inevitably ratchet tensions between Loyalists and Republicans as both sides are forced to reconcile with the fact that their hard-fought status quo cannot remain intact. Without such guarantees in place, the delicate peace that exists between neighbors, still literally walled off from each other, could quickly deteriorate. With Brexit acting as a potential catalyst for a return to the Troubles, May and the British people will have to confront the possibility of renewed insurrectionism and violence at a time when their country is facing one of the most complex break-ups in history.

With memories of bombs in Canary Wharf and Arndale Shopping Center weighing on the former Home Secretary’s mind, her calculus as she prepares for negotiations with Westminster, Holyhead, Stormont, and Brussels will be heavily impacted by the outcome of the Northern Ireland question. The implication for businesses and investors who are wary of a potential “hard Brexit” for the UK is three-fold. Firstly, should Brussels remain steadfast and insist that the preservation of a Common Travel Area and “non-foreign” status between the UK and Ireland is contingent on the UK’s continued participation in the single market (read: freedom of movement with the EU), then May’s “red line” may find itself a bit more flexible.

Secondly, if the UK is forced to abide by freedom of movement in order to secure the continuation of its special relationship with Ireland, and it chooses to do so, then it would castrate the biggest incentive for the UK to extricate itself from the single market. In effect, the best Leave supporters could hope for is a “soft Brexit”, one that has the UK retaining its rights within the EU in all but name.

Lastly, should the UK instead go for a “hard Brexit” that requires it to revise its immigration and border laws for Irish citizens in an post-EU world, then it will have to face potential upheaval and resistance in Northern Ireland. Social instability at the same time as political and economic instability will likely leave Westminster convinced that a more nuanced approach to the Brexit outcome will be necessary.