I went to see Christopher Nolan’s Dunkirk without thinking. “Dunkirk” is a name I have always lived with. As a child in England, while the war ended, my family fed me “Dunkirk spirit” as if it had been the turning point of the conflict, as well as an exemplary display of humble English greatness. I had the broad factual outline; I knew the numbers, the more or less 330,000 men taken off that stricken beach.

But as I was watching the film, I was counting in another way. Dunkirk was evacuated in the first days of June 1940. And now the possibility dawned on me – I was born on 18 February 1941 – that I would have been conceived in that dark moment, probably just before the predictable doom that came with the defeat of the British Expeditionary Force in France and Belgium.

And that’s a mystery, as if one’s parents are ever anything else – they are so set on their secrets. You see, they did not get on: my mother admitted that later; my father never addressed the matter, as if “getting on” was outside his range. I never heard him say he loved anyone. Not that love is an automatic, speaking companion to sex and conception.

They must have had sex – whatever they called it. They were in their 30s still, and they were both attractive and lively. I did gather later that my mother liked sex. She had encountered it somewhere. My father said not one word to suggest he knew about it. I can’t believe he and Mum ever talked about it. But he’d told my mother that he wanted no children, and that if she got pregnant and delivered a child, he would leave her.

She did get pregnant (just that once), and he did leave her. After she had been in the nursing home the 10 days required after delivery in 1941, she came home and he was gone.

So at some point, as the British army withdrew in Europe, it happened. But my “it” occurred as the British command foresaw the likelihood of losing an entire army, and thus the war. I asked my mother what she thought at the time and she said she had hoped for the best. Good enough, and thank you, Mum. As for Dad, well I can believe, in his silent darkness, that he was saying “what the hell”. Why worry about offspring if it’s the end of the world? Did he count on the worst?

Or was there even some stray hope in him for furthering the human race in its most forbidding time? Was he defying fate, as well as Hitler?

One wants to think one’s father is single-minded and in charge, even if he was a bastard about it. But why should he not have been as confused as Hitler? History can handle Hitler being insane, but confused is harder to deal with. Still, no matter that this is omitted from the film: there are strange mysteries attached to Dunkirk.

The Germans argued over what to do at the Dunkirk beaches. The army commander, Von Rundstedt, called for more of the same, an armoured infantry thrust that would obliterate the British and be an emphatic victory. Hitler hesitated. He was not sure tanks could function on beaches. He took Göring’s advice that the beach would be better strafed from the air. Was it simply a matter of military indecision, or was Hitler inclined to let the British off? Was he more certain that the Soviet Union was his true enemy? Was he even contemplating a peace with Britain that would free his adventures in the east? Was he nervous of that Channel crossing? He never attempted it. As late as 1945, he liked to say that he had given Britain “a sporting chance” in 1940.

Perhaps Hitler needed leeway in his own mind. My father did leave my mother – he went from south London to beyond the north of the city and he lived with another woman. But he came home every other weekend and he took me to see Chelsea FC, Surrey county cricket club, Reg Harris cycle at Herne Hill and Sydney Wooderson run at White City. There were several years in which I loved him, and then a few more when I made myself look away from his odd routine. He was a tyrant, I learned to see, most of all in his refusal to discuss anything important. But many tyrants are muddles.

So I was raised to think of Dunkirk as the time when ordinary people in England got out their boats, sailed across the Channel and brought back four or five, or 45, exhausted soldiers. When I was a child, I met a man who had taken his small yacht to save a few and he recalled that he had just gone, without orders or guidance, in a very unofficial British curiosity. Mark Rylance stands in for such fellows in Dunkirk and he does a good job of it, as if to ask aren’t most Englishmen handy supporting actors, even if they tend to be tightlipped about it to the point of comedy?

• Warner Bros: The Story of an American Movie Studio by David Thomson is published next week by Yale University Press, £16.99.