It remains a defining image of last year’s US presidential election. Khizr Khan, speaking at the Democratic national convention with his wife, Ghazala, by his side, produced a copy of the constitution from his jacket pocket, held it up for all to see, and offered to lend it to the then Republican candidate Donald Trump. It also remains the most eloquent response to Trump’s bigotry.

Khan, who grew up in a small village in Pakistan, was talking about the sacrifice his son Humayun had made for his country – America. Humayun was killed aged 27 in Iraq 13 years ago, protecting his men from suicide bombers. He is buried at Arlington cemetery, Virginia, alongside so many other war heroes, and was posthumously awarded the Bronze Star and Purple Heart.

“We are honoured to stand here as parents of Captain Humayun Khan and as patriotic American Muslims,” Khan began. He continued: “Donald Trump, you’re asking Americans to trust you with their future. Let me ask you, have you even read the United States constitution? I will gladly lend you my copy. In this document, look for the words ‘liberty’ and ‘equal protection of law’.” Still addressing Trump, he asked: “Have you ever been to Arlington cemetery? Go look at the graves of brave patriots who died defending the United States of America. You will see all faiths, genders and ethnicities. You have sacrificed nothing – and no one.”

Now Khan has written a memoir simply, and pointedly, called An American Family. It tells the story of the Khan family, but it is also a universal story of migration – struggle, hope and achievement. Khan, one of 10 children, was 24 when he left Pakistan, first for Dubai then eventually the US. He faced many difficult decisions along the way – leaving the family behind in Houston while he fulfilled his dream of studying law at Harvard; not telling Ghazala that he was so broke after graduating that he had to sleep on a park bench until he got his first wage packet.

But the most difficult decision was choosing to talk at the Democratic convention. The Khans had always been a private family, and now they were contemplating sharing their grief with the whole country for a political end – to help the Democrats. The irony is that Khan had spent his early days in the US idolising the Republican president Ronald Reagan.

For Khan, politics had been a private matter. Then, in December 2015, just after Trump had made his speech calling for a ban on Muslims entering the US, he was called by a journalist who asked what he thought about it. Khan discovered he had plenty to say about Islam and US patriotism – not least about how his son Humayun had exemplified American values. The interview was published under the headline: “The father of a Muslim war hero has this to say to Donald Trump”.

When Khan, now aged 67, was then approached by Hillary Clinton’s campaign, he sought advice from his two surviving sons. Both said he shouldn’t talk at the convention. “They said you will be fair game for criticism, your credibility and reputation will be questioned, you will be maligned.”

But something nagged away at him and Ghazala. They talked to youngsters who asked whether it was true that if Trump were president he would ban Muslims from the US. Khan had carried a copy of the constitution around with him since 2005 (when he started to present visiting cadets paying tribute to Humayun with a copy of it).

“I would pull it out, and read section one of the 14th amendment – that citizens have equal dignity and equal protection of the law – to hearten these children, but they remained concerned. Some wouldn’t go to school and some wouldn’t eat properly. Their sleeping habits changed.”

How could he look the children in the eye and tell them he shared their concerns if he wasn’t prepared to talk at the convention? They thought about the sacrifice their middle son had made for their country. “We would sit in the room where Captain Humayun Khan’s picture and his memorabilia is kept and we would look at that and say: ‘What would he do?’” There was only one choice.

Did Khan worry that speaking might reignite their grief? No, he says, because it had never left them. The couple had made a conscious decision to keep Humayun alive in their memory by establishing a scholarship for him at the University of Virginia, where he had studied, and by inviting cadets to their home.

Even today, he says, they have not fully accepted their son’s death. “Pardon me if I lose my composure,” he says, quietly. “Whenever we encounter soldiers in uniform we see the amazing dignity in them, and it is a reminder of how Humayun conducted himself, how he talked to others, and we begin to feel he has just appeared from his room with his smile and amazingly graceful way.” His voice wobbles. “If you ask Mrs Khan this question, she says to me: ‘Why do people tell me he is not here? He is here, he is with us. I feel him every day.’”

Khan asked Ghazala if she would speak first at the convention. “She just looked at me, and I knew that was a no. Ghazala could not even enter the room with Humayun’s picture in it for seven years. It was our youngest son’s job to clean that room. She said: ‘You think I’ll be able to even stand if his picture is on the screen?’ I said, ‘OK, just stand with me and hold me. I will hold you and you hold the podium.’ When we got off the stage you can see I’m trying to put my arm under hers to make sure she doesn’t fall.”

An American Family gives the impression that Ghazala was by far the more active parent – for one thing, Khan seemed to be working all hours. “That is so true,” he says. “I say our three sons are a true reflection of Ghazala’s strength, care and love. Between the two of us, Ghazala has been the better person in the family.” Did he feel guilty for not being there enough when the boys were young? “Yes. We went to visit our eldest Shaharyar, who has three children now. There was a guest in the house and she asked: ‘What would you do differently raising your children now?’ I said: ‘I would raise my children as Shaharyar is raising his.’ He is there every night to give them baths. Whenever they have a bad dream they run to their father in the middle of the night. I wish I had been wise enough and able enough to do that for my sons.”

Perhaps devoting so much time to Humayun’s memory is his way of making up? “I don’t know that it is deliberate … Maybe,” he says.

As his sons predicted, there was fallout from the speech. Trump questioned why Ghazala, who has a master’s degree in Persian studies, had stood by her husband silently as he spoke. “She had nothing to say … maybe she wasn’t allowed to have anything to say,” he said.

How did Khan feel about Trump’s attack? “As a spouse, and knowing how mistaken this person is, I was upset. I was angry that this person again proves his ignorance to me. Ghazala responded to it so well. She said: ‘He doesn’t know me, and he doesn’t know Muslim women, and he doesn’t know how a grieving mother would feel if she’s asked to speak at such a moment.’ She answered that question better than I could. But I was angry.”

Was it ignorance or Islamophobia? “It was intentional for political gains. And it is the characteristic of a person who is arrogant and ignorant and practises divisiveness. Every statement he has made since being elected president has been divisive.”

You never mention Trump by name, I say. “I don’t dignify him by calling his name. He is president of the United States, but I don’t call him that.”

Khan has received hate mail since the speech. “We get some bad letters and emails – threatening and really disrespectful.” But, on the whole, he says the response has been so positive. “We have also received the most wonderful cards and letters. Thousands and thousands of them.” So many people have told him that something in his speech resonated with them.

Khan was recently recognised by an airport check-in worker during his book tour. “The fellow who was checking me in at the airport recognised my face and name. He came out and gave me a hug and said: ‘I also lost my son, Mr Khan. Both you and I know that hole in the heart is never filled. We just get used to living with it, so be strong, and you have my support and my prayers, and he went back to his counter and dealt with the rest of the passengers.’” He pauses. “It is true. That hole in the heart always remains.”

The following is an extract from An American Family by Khizr Khan

‘He saw something. Maybe a look on the driver’s face. Captain Khan turned, yelled for everyone to hit the dirt. The taxi exploded’

My son was in a coffin.

I saw him, looked at him, stared at him. He was my son. My beautiful boy.

For days, I’d continued to convince myself the army was wrong. Even on the drive to the funeral home, I told myself there had been a terrible mistake. An army casualty officer, a captain, drove me, and only me; I was still the stoic one in the family because I had to be. I knew that when we got there, when we saw the body in the casket, I would shake with relief and gratitude. Then I would tell them to go, hurry, go to the family that needs you, the family of this poor, lost soldier.

The captain stood behind me as I approached the casket. He stood at attention, saluting.

A sudden tranquility interrupted my thinking. I nodded slightly. “Yes,” I said. “That’s my son.” Because it was.

It is a cliche, but he looked peaceful, as if he was sleeping, perhaps even dreaming of something pleasant. There were no marks or blemishes on his smooth, brown skin. The dimple in his chin was unmarred. The only unusual detail was the bandage, a bright white stripe of gauze stretched across his forehead, covering the wound where a piece of shrapnel had hit him above his left eye.

Humayun had been deployed to a forward operating base called Camp Warhorse. Tuesday, 8 June, was his day off, but he went out to the main gate that morning to check on his soldiers, who were responsible for, among other things, security. It was early, about eight o’clock, what everyone called rush hour because of the Iraqi civilians arriving for work. The camp employed about a thousand locals, which was both practical – work needed to be done – and strategic: giving people jobs improved the local economy, which in turn improved living conditions, and people in general are less likely to see as enemies the soldiers who are giving them, or their fathers or mothers or brothers, a pay cheque.

The Americans were never meant to be the Iraqis’ enemy. That would become easy to forget as the war dragged on, but the point of invading Iraq was supposed to have been liberating the population from a despot, not subjugating it to American imperialism. It was right there in the name: Operation Iraqi Freedom.

Still, by the late spring of 2004, an insurgency had taken root, and Diyala province was one of the seeding grounds. Camp Warhorse had been repeatedly attacked. Security was abundant and tight. There were a series of barriers and gates, a course of deliberate obstructions, leading to the main road. Vehicles were stopped and inspected, mirrors sweeping the undercarriage, the engine compartment and trunk poked through. Civilians parked in a fenced lot set well back from the perimeter of the camp proper.

Vehicles coming into the base had to pass though a long canyon of blast walls laid out with sharp turns that forced drivers to manoeuvre through them at a crawl. Towers flanked the path, .50-calibre machine guns tracking anything moving. Sometimes, rarely, a car slipped through, the driver refusing to stop, ignoring the warnings.

On that Tuesday morning, it was an orange taxi with a driver and a passenger. One of the guards notified Captain Khan. Humayun could have told his soldiers in the towers to put a few rounds through the windshield and into the engine block. He could have ordered the taxi destroyed, the driver and his passenger killed. If the vehicle was a threat, a short burst of gunfire could have easily stopped it.

But what if the taxi wasn’t a threat? What if the driver was confused, didn’t understand what those foreigners were yelling, didn’t know if all the arm-waving meant stop or go or turn around? What if he was just trying to get some poor guy to work so he could earn a few bucks for his family?

Humayun still could have told his soldiers to shoot him. That would have been perfectly legal. It also would have been prudent. It was hardly his fault that some Iraqi driver got confused. It had happened before, right in that very canyon of blast walls, happened a hundred times, a thousand times, all over Iraq. People die in wars. Mistakes, awful and tragic mistakes, happen.

Captain Khan told everyone to stay down. He started walking towards the taxi, his arms up, a universal sign to stop. He wasn’t going to kill a man by mistake. He believed in his mission, the overarching point of which was to bring freedom and safety to people in a country where they’d had neither. He would tell Iraqis that, directly and explicitly. “We’re not here to hurt you,” he would say. “We are here to help you.” He and other officers had organised local men to patrol Baqubah, the provincial capital, paid them $5 an hour to go out with American soldiers, make them part of their own security, their own destiny.

How much damage would one dead civilian cause? How much of that work would be destroyed?

He took a few more steps, arms still up. I was told he took 10 steps, but I don’t know if anyone actually counted.

He saw something. Maybe a look on the driver’s face, maybe the explosives packed around him. Captain Khan turned, yelled for everyone to hit the dirt. The taxi exploded. The concussion killed him instantly, almost certainly before the shrapnel even hit him. Two Iraqi civilians close to the car were killed, too, and 10 soldiers were wounded. And now my son was in a coffin, a white bandage on his forehead, peaceful, at rest.

The army said he was a hero, awarded him a Bronze Star and a Purple Heart. Who knows how many would have died if the taxi had gotten closer to the gate? How many other fathers’ children weren’t in coffins? Dozens? Does the number matter? Is one enough?

But there was also this: My son was dead because he was trying to make sure a stranger wasn’t killed by mistake. He stayed true to the shape of his heart. Humayun Saqib Muazzam Khan, a most fortunate one, destined for greatness, a king who would shine among the heavens. He was, and I believe he does.

I studied his face, so young, so kind. I did not think of those things in that moment. I’m not sure what I thought of, or if I thought at all. Pride can be a balm, but it is not immediately effective, and it never, ever heals the wound.

• An American Family: A Memoir of Hope and Sacrifice by Khizr Khan is published on 24 October (Sphere, £20). To order a copy for £17, go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min. p&p of £1.99.