Boris Johnson today announced that the Conservative government intends to propose a hard border between Northern Ireland and the Republic, something that Theresa May, to her credit, went to enormous lengths to avoid in negotiations over Brexit. There is no doubt this proposal, while apparently welcome to the DUP, will be rejected by the EU and by the majority of people in Northern Ireland. This move and Johnson’s earlier suggestion, before becoming prime minister, that the border in Ireland was analogous to the border between Islington and Camden, makes me wonder if he understands how high the stakes are.

I have worked for more than 30 years trying to help end armed conflicts, from Northern Ireland to Colombia. One thing that strikes me is how often countries slip into them by accident, not because the leaders want to start a civil war but because they get carried away with their cause – and feel justified in doing absolutely anything to win, without fully thinking through the consequences.

They start with intemperate language. With the feeling the other side is unreasonably stopping you securing your legitimate aim. Every time one side wants something it is blocked by the other. The anger usually derives from different causes, unfairness and limited opportunities, but the anger is directed against the other. Each side doubles down rather than compromising and raises the temperature still further. Each side has a narrative of grievance, and the end comes to justify the means.

I don’t suppose the Rev Ian Paisley intended that his over-the-top rhetoric and inflammatory march on a Sinn Féin Office in 1964 to pull down the Irish tricolour would contribute to the start of a 30-year civil war. He probably thought he was just protecting the Protestant people’s democratic rights as the majority, against the Catholic threat to their identity, and being tough in making the case for his side’s political and religious rights.

Pro-independence demonstrators in Sabadell, Catalonia, protest against the arrest of Catalan activists. Photograph: Emilio Morenatti/AP

But then the politics turn septic. Supporters on both sides are so wound up by the politicians’ rhetoric that they start taking things into their own hands. The first victim of the Troubles was a young Catholic, John Scallion, shot outside his house in west Belfast in 1966 by a Loyalist gang. He was killed for being a Catholic. Within days two more had died, and the Northern Ireland Troubles had kicked off. It took more than three decades to end them.

Of course, if you had told anyone involved in any of these conflicts at the beginning that it was going to end in serious violence they would have laughed at you. But look at what has happened even in Catalonia in the last two years. All sides were determined that it should be a purely peaceful political disagreement and there were no comparisons to be drawn with the decades-long ugly war in the Basque country between ETA and the Spanish government. But the rhetoric became radicalised and both sides said and did things they probably now regret.

Then in 2017, the Guardia Civil used force to try to stop the referendum, and last week they arrested what they termed suspected separatist bomb-makers. When the Spanish courts hand down sentences on the Catalan politicians who championed independence in the next few weeks, it is almost certain that there will be serious violence. A dispute about identity that everyone assumes will always be peaceful can turn ugly very quickly, however civilised the country.

Supporters start calling for riots. Then it is too late. There has been blood and there has to be revenge

What is true for Catalonia or Colombia or Northern Ireland is just as true for Britain. I don’t suppose it is Johnson’s intention to lead the country into its first civil war since 1651 (when that one started, incidentally, in 1642, it was a result of an abuse of the royal prerogative), any more than it was Paisley’s in 1964. But we may look back and discover some years down the line that he has done just that.

He certainly seems intent on turning Britain into two warring factions, determining our identity as Brexiteers or remainers. Of course there are economic and social causes behind the anger, particularly stagnant wages and limited life choices for a large section of the population, but that anger can be – and now is being – directed in support of English nationalism. Instead of uniting the country, the prime minister appears determined to divide it for his own political purposes. Just like Paisley, his determination to win the argument could have disastrous consequences.

Conflicts really take off when one side starts issuing verbal threats. With the IRA the threat was: if you don’t agree to a united Ireland, you won’t be safe. Now, according to Johnson, if you don’t agree to my Brexit, you won’t be safe.

Supporters start calling for riots. Then it is too late. There has been blood and there has to be revenge. The ghastly cycle of violence begins. Killing by one side justifies killing by the other. Our grievances are more important than yours. Our claim is greater than yours. You started it, so we are justified in responding.

Once a war gains momentum, no one really remembers the original cause, just that it was the other side’s fault. Even if one side manages to win and dictate a solution, the defeated side won’t forget. They will just look for their opportunity to fight back. The only way to end it is to arrive at a compromise that both sides can live with, reluctantly: no winners, no losers.

God willing, we will probably stop our descent down the slippery slope we have embarked on before we get to these desperate straits. But those playing with fire need to bear in mind the potential unintended consequences of what they do.

One thing the world discovered long ago: civil wars are a lot easier to start than to stop.

• Jonathan Powell was Tony Blair’s chief of staff from 1995 to 2007 and is now director of Inter Mediate, an NGO working on armed conflicts around the world