BELFAST—I’m driving across Europe’s most divided city, where politics is existential and fear often only a few streets away.

We’re heading west toward the River Lagan from the largely Protestant east, the flags of illegal paramilitary groups hanging limply from lampposts. Sitting beside me in the car is someone who describes himself as “an active loyalist”—loyal to the British Crown and state and opposed to a united Ireland—but, like other unionists I spoke with, asked not to be identified for fear of retribution. He is a member of the city’s Protestant working class, which has united in anger at Prime Minister Boris Johnson’s prospective Brexit deal with the European Union, principally because of the de facto customs border that it proposes between Northern Ireland and the rest of the United Kingdom, in order to avoid one with the Republic of Ireland.

Johnson called a general election seeking a mandate to deliver his deal. In loyalist parts of Belfast, they are determined to show he has no mandate from them.

To my passenger and every other Protestant unionist I met, in Belfast first and then in rural Fermanagh, near the Irish border, Johnson’s plan was a betrayal. Not just any betrayal, in fact, but a betrayal of the very issue most fundamental to them: the union between Northern Ireland and Great Britain, whose preservation had been the source of a decades-long civil war before the 1998 Good Friday peace agreement brought it to a close with an explicit recognition from all sides that Northern Ireland would stay in the United Kingdom as long as a majority of its people wanted it to. Again and again in my conversations here, I was told that Johnson’s Brexit deal amounted to a betrayal of this peace settlement—an “appeasement” of Irish Republicanism and its implicit threat of violence should any land-border controls be erected with the Republic of Ireland.

This perceived betrayal goes to the heart of the sectarian fissure that continues to dominate Northern Ireland: between the mostly Protestant unionists, who largely identify as British, not Irish, and the largely Catholic nationalists, whose identity runs the other way and who would like to see Northern Ireland united with the Republic of Ireland. They are two communities in a zero-sum contest over which country to belong to. From the late ’60s to the mid-’90s, they were at war—the Irish Republican Army attempting to shoot and bomb their way to Irish unity; loyalist paramilitaries, fearful of British weakness (but whose leaders are now known to have colluded with British forces), responding with their own violence to stop such an outcome. While the IRA killed far more than any other group throughout the conflict, by the early ’90s, loyalist paramilitary organizations were carrying out more murders than they had in the past in an effort to terrorize the Republican community into ending its campaign.