No one knows better how fragile peace is in Northern Ireland than Jonathan Powell, the man who wrote the Good Friday Agreement.
No one knows better than he that what politicians downplayed as “The Troubles” was in reality a bloody, bitter, 30-year civil war which cost thousands of lives.
And no one knows better how easily the balance could tip from uneasy harmony into another violent conflict.
Today he warns Boris Johnson that his fanatical drive towards a no-deal Brexit is recklessly putting peace at risk.
To rip up a done deal with the EU and replace it with his Internal Market Bill will wreck international trust in the UK and lead to a hard border between north and south in Ireland.
That will give the island of Ireland’s festering terrorism, at the moment largely dormant, the excuse it needs to swap ballot boxes for bombs and bullets.
Mr Johnson’s first duty as the UK’s prime minister is to keep people safe. The path he is embarking on is one of negligence bordering on the criminal.
MPs, including many Tory ones, are appalled by what the PM is doing. So is Tony Blair. And two former Tory PMs, John Major and Theresa May. Mr Johnson chooses not to hear them.
But there is still time to listen to the warnings from Mr Powell, who knows Northern Ireland far better than him.