We don’t remember the near-misses. Everyone can tell you where they were when they heard that the Twin Towers had been attacked in 2001. Britons can also recall the 2005 London Tube attacks, in which 57 people died. But almost no one remembers Sept. 15, 2017 when, through the merest chance, another bomb attack on the London Underground resulted in only minor injuries.

Time divides into branches, and we, inhabiting the branch where the bomb did not tear devastatingly through a rush-hour Tube compartment, quickly move on to other things. But it is worth pondering for a moment the other version. The bomber, Ahmed Hassan, had packed five pounds of shrapnel – knives, screwdrivers, bolts – around his explosives in a bucket. The train was heaving with people. We could easily have seen a worse massacre than in 2005, perhaps surpassing even the 67 Brits who died on 9/11 – still the single worst loss of life suffered by British nationals in any terrorist attack.