If you live anywhere but Northern Ireland, July 12 is just another day of the week for you. If, however, you happen to live in Northern Ireland, it is a national holiday, but one that is celebrated by only half of the population. You might say it is the Fourth of July in reverse—a celebration of remaining part of, rather than seceding from, the British Empire.

I was barely aware of this holiday myself until I visited Belfast last month. It commemorates the victory of William of Orange, the Dutch-born, Protestant monarch who had just taken over to the British throne, over the forces of the deposed Catholic King, James II. The Battle of the Boyne, fought about 30 miles north of Dublin in 1690, ended once and for all any hopes that a Catholic could sit on the British throne and ensured a Protestant ascendancy not only in England but also Ireland—then an integral part of the British Empire.

Today, most of Ireland has long been an independent republic, but six counties in Northern Ireland remain under British sovereignty. Ever since Michael Collins agreed in 1921 to allow Ulster to remain under the Crown as the price of independence for southern Ireland, the division has been resisted by Catholic die-hards. It has been just as adamantly defended by Northern Irish Protestants, who style themselves as Orangemen after good King Billy.

Hardliners in the Anti-Treaty IRA fought a losing civil war in 1922-1923 against their own erstwhile commander, Mick Collins, in an unsuccessful attempt to overturn this compromise. They lost the war, but killed Collins.

Another insurgency, known as the Troubles, was launched by the Provisional IRA in 1969, sparked by the complaints of Northern Irish Catholics that they were being discriminated against by the dominant Protestants. The result was a long-running, low-intensity conflict that led to the deployment of the British army to Northern Ireland and claimed more than 3,500 lives. The fighting finally ended with the Good Friday Accords of 1998, which instituted power-sharing between Catholics and Protestants in Belfast.

Save for a small splinter faction known as the Real IRA, most of the Irish Republican Army has given up political violence and turned to nonviolent action via its political party, Sinn Fein. The Protestant paramilitaries, notably the Ulster Defense Association, have also largely stopped carrying out attacks against Catholics. Both the Protestant and Catholic paramilitaries have been implicated, instead, in drug-dealing, extortion, and other criminal schemes. An uneasy truce prevails, but the signs of discord remain very visible to anyone who drives across Belfast.

In the Protestant, working-class areas one still sees giant murals honoring martyrs of the Ulster Defense Association—terrorists such as Stevie “Top-Gun” McKeag, who got his nickname for murdering Catholics, both civilians and IRA fighters. He died in 2000 of a drug overdose. A particularly chilling mural nearby depicts two Ulster Defense Association fighters, their heads covered in ski caps, pointing assault weapons at the viewers. This is all too reminiscent of the propaganda that ISIS puts up in areas under its control.

Meanwhile, in Catholic, working-class neighborhoods, there are competing monuments to martyred IRA fighters. Most prominent of all is a giant painting on the side of the Sinn Fein headquarters honoring Bobby Sands, the IRA leader who in 1981 starved himself to death in a British prison to protest Margaret Thatcher’s determination to treat IRA prisoners as ordinary criminals.

Separating the Protestant and Catholic neighborhoods are giant walls that remind me of similar walls erected in Baghdad in 2007 to separate Shiite and Sunni areas. The gates between the neighborhoods are open during the day but typically closed at night to prevent hot-heads from either side from making mischief in the sectarian cantonment next door.

Tensions will run especially high on July 12, when Protestant Orangemen march to commemorate the Battle of the Boyne. The night before, Protestant communities light giant bonfires to which they consign the flag of the Republic of Ireland and various other Catholic and Irish nationalist symbols. More than a month ahead of time, I already saw vast piles of wood being stockpiled in Protestant areas, getting ready for the sacred day.

In the past, Protestant marches, especially through Catholic areas, have led to riots and violence. That is unlikely to happen now, but you never know. There’s a reason why the police in Belfast (once known as the Royal Ulster Constabulary, today simply the Police Service of Northern Ireland) still patrol in armored cars.

Oddly enough, given the lingering tensions in Northern Ireland, there is absolutely no barrier whatsoever between its territory and that of the Irish Republic. The only way you know that you are crossing from the United Kingdom to the Republic is that the speed limits change from miles to kilometers and the cell phone providers change, too.

There are fears now that, with Britain leaving the European Union, border controls might be forthcoming, but this is one issue—one of the few—that unites Catholics and Protestants. Both Sinn Fein and the Democratic Unionist Party, the dominant Protestant party in Northern Ireland, oppose the erection of any border checkpoints.

What’s striking to me is how much suspicion and animosity still lingers between Catholics and Protestants—all the more so considering that few on either side are remotely pious. The old joke has it: Aren’t there any atheists in Northern Ireland? The punch line: Sure, there are Protestant atheists and Catholic atheists.

It’s easy to think there is something wrong with the Northern Irish, but increasingly I wonder if their situation isn’t merely a somewhat more aggravated form of the tribalism that is increasingly visible across the entire world, from the Philippines to Italy, to pick two countries at random that are experiencing significant secessionist movements (by Muslims in the Philippines and northerners in Italy).

We see it even in the United States, where Republicans and Democrats increasingly lack a commonly agreed upon set of facts and a common vocabulary: Are Trump opponents the brave Resistance or contemptible Snowflakes? We are more disunited than ever—or at least more than we have been in a very long time.

A visit to Northern Ireland is a bracing lesson in what can happen if divisions—whether ethnic, racial, religious, regional, or ideological—spin out of control. It’s also a reminder of how hard it can be to patch up civil society once its foundations disintegrate.