In 2007 I was in Arlington National Cemetery [the US military cemetery] with my wife, Janine Altongy, to take photographs for my book War Is Personal, about Americans affected by war. We were there to photograph Paula Zwillinger, a New York resident who had lost her son, and a young girl named Kayley Sharp, who was visiting the grave of her father. At the time, section 60 – where Afghanistan and Iraq veterans are interred – was growing.

As I wandered about those stones, there was no way to escape the crushing weight of the grave markers, row after row, line upon line of them, running to the horizon, rising and falling with the land. Without exaggeration, this cemetery is awe-inspiring, beautiful and kind of frightening. It was a lovely day, but I found it oppressive. Depending on your political point of view, these endless hills of the same stones show either democracy or the cost of war – because everyone is rendered equal, but not by the best of circumstances.

Facebook Twitter Pinterest Humayan Khan’s grave at Arlington national cemetery. Photograph: Mark Wilson/Getty Images

That morning, there was a slender, stern-looking man in section 60. At first, from a distance, I thought he was a Protestant priest because he was dressed so somberly. From the side of the headstone I was on, you could only see a number, not the crescent moon and star that showed it was the grave of a Muslim soldier. It was only when I saw the man’s posture that I realised what his faith was.

I suspect from looking at the picture now that I took it before I spoke to him. I would have watched him to ascertain whether he would mind – if there is a sense of discomfort, you don’t take a picture like this in a cemetery. Afterwards, I remember he didn’t give me his name, but just said his son was buried here. All the people we met that day would do that – they would not speak about themselves, only the person they were here for, often as though the person was still alive.

He was a very subdued, almost fearsome-looking man, so I didn’t know what to expect. When he spoke, he was very kindly and quiet. You could tell he was a very private man. I didn’t know who his son was then – Humayun Khan, an army captain who was killed walking towards a suicide bomber after ordering his men to “hit the ground”. We didn’t include him in the 15 stories – the picture of him was more of a melody for the piece we were doing, not the central focus. Later that day, I took pictures of the headstone but I didn’t use them. It felt too easy to make the point that there are all kinds of people buried at Arlington. It sounded trite – a stupidly obvious thing to say. But I guess now it needs saying.

Last week, when I heard Khizr Khan’s speech, the memories of that day came back. It was a shock to see him on the stage, but it wasn’t a surprise because he had exuded intelligence. His incredible dignity had rung a bell in my mind. When we posted it on Facebook, we didn’t realise people would be drawn to a simple photograph of a quiet man in mourning.

What did I think about Trump’s comments [the Republican candidate claimed Clinton’s speechwriter was behind Khan’s speech, and that his wife, Ghazala, did not address the convention because she was “not allowed to have anything to say”]? Khizr Khan’s presence was so informed, and Trump’s comments so uninformed, that it was frightening. What a lousy election this is. It’s so exasperating that Trump can say these things, and that they bounce off so many of my fellow Americans – they should be mortified by them.

Facebook Twitter Pinterest Khizr Khan and his wife, Ghazala, at the Democratic national convention. Photograph: Shawn Thew/EPA

The fact Trump made remarks about Ghazala Khan’s silence being tantamount to subservience is so wrong. Her husband might be hyper-articulate, but not many of us could get up on a stage and talk about our sons if they had died – I sure as hell couldn’t. I wrote another post on Facebook about how grief silences us. Kayley, the young girl I photographed at Arlington the same day I photographed Khizr Khan, didn’t emit one single sound that November day when crouched down by her father’s grave. And there were no tears; in my picture, her eyes are downcast, partly shrouded by windblown strands of her hair. Hers, one could see, was an internal, unspoken, unspeakable grief. I would come to learn later from Jen Henderson, Kayley’s mother, that Kayley pretty much stopped speaking to anyone but her, upon learning that her dad, Sgt First Class Christopher Henderson, wouldn’t be coming home from the war in Afghanistan.

As for Ghazala Khan, the question to ask is not why she was silent, but where she found the strength to go out on to that vast convention stage, in front of millions of viewers, while feeling the way she felt about her much-loved son, her fallen son. When she is still in mourning.

This picture is so simple it shouldn’t even be remarked upon; it’s a guy mourning his son who happens to be a Muslim American. I have been disappointed in my country’s propensity for being involved in other people’s wars. But in its best form – maybe in the idea more than reality – there is an American spirit that has helped the world. Away from the ugliness and rhetoric of the election campaign, we are still holding together, and I hope this little picture speaks to that.