I was 37 when I first asked whether my grandfather had been a member of the Nazi Party. The question had often lurked in the background of our conversations, and my father answered without further ado: Yes, Grandfather had been in the Nazi Party, because he had run the family business with his brother, who was a freemason, and therefore ineligible to join. “Someone had to join the party.”

Right question, wrong answer? I couldn’t ask my grandfather. He had died of a heart attack at home in Kassel in 1944. So I was late in learning that the old stories were far from being over. That everything we had learned at school about repression in the Adenauer period (1949-1963) had affected us, the generation swamped by history, even more directly than we thought.

Through my research I discovered the story of denial, lies and subterfuge lasted to the present day. While the US government had held captured Nazi records - such as the party’s central membership file - the German government had used every available means to delay the return of the Nazi archive for half a century until the last top politician with a Nazi file had retired.

Searching archives in Germany and the US, I found lists of high-ranking German politicians whose Nazi membership files had been secreted between the 1960s and early 1990s. Many of this generation kept quiet about their connection to the Nazi Party, or denied it, or pushed it to the backs of their minds and forgot all about it. When I confronted them about their party membership, most of them flat out denied having applied to join Hitler's party - even in the face of overwhelming evidence.

In my book Post-War Lies I tell their hitherto unknown story, from the Third Reich to the post-war de-Nazification process and into the present. But my main interested wasn’t just exposing the youthful sins of a bunch of old men. I wanted to dig deeper and ask questions about about historical truth and human honesty. For that, I had to start in my family.

My father was born in 1927. He's 87 now, the same age as grandees of the old Federal Republic such as Günter Grass, Hans-Dietrich Genscher, and Martin Walser. My grandfather, Walter Herwig, was born in Kassel in 1880, almost a hundred years before me. As they both came late to fatherhood, the Herwigs skipped a generation, and I was born within earshot of the 19th century.

My father came into the world in the Weimar Republic, but his childhood home was rooted in the German Empire. And so my father had one foot in the Kaiser era, yet had to march into the Third Reich with the other. A photograph from the 1930s shows little Günter Herwig on the street, smiling dutifully into the camera and raising his right arm in a Hitler salute. Beneath it in the photo album is a note written by my grandmother: "His first Heil Hitler. " Did he know what the gesture meant?

However, he didn't take any interest in marching, and was hauled up before a Hitler Youth court in 1943 for having gone awol after the roll call at the beginning of a march. He liked theatre, but not the theatre of the Brownshirts. According to the penalty ruling of the Kurhessen Hitler Youth, he had broken ranks with the march formation because he had sprained his ankle during an amateur dramatics performance. His unauthorised decampment was punished leniently: a warning "for the duration of the war".

Indifference is a powerful force that is often underestimated, even by dictatorships. If everyone had been like my father, the Nazi State might soon have collapsed from a lack of interest on the part of those involved. But that wasn’t what happened, and the Third Reich needed more heroes than the few students, workers, and officers who lost their lives in the courageous struggle against injustice, and who have since had to act as an alibi for “the other Germany”. Brecht was right: “Unhappy the land that needs heroes.”

Young Günter Herwig was not swift as a greyhound, tough as leather, or hard as Krupp’s steel - as Hitler Youth were expected to be. Instead, he was pink, soft, and complacent. So my father adopted a completely unheroic form of passive resistance in the Third Reich, and there was very little the powers that be could do about it. They met open rebellion with reprisals, concentration camps, and death sentences. But no state could be made with lazy “national comrades” like him, let alone a “Thousand-Year Reich”.

My father didn’t like to trouble the past, but the past troubled him. Whenever there was something about Hitler or the Holocaust on television he would change channels. He preferred animal documentaries. He never had to experience war at the front. But he did witness the destruction of his hometown of Kassel, which was reduced to rubble by Allied bombs in 1943; the corpses in the street outside the bunker in the Weinberg part of the city; the sirens; the firestorm.

I often asked him about his memories of those times. When he was an adolescent, did he know what was happening to the Jews? “They lived in a different part of town,” was his answer. But he also told me how surprised he was when his Jewish pediatrician disappeared one day.

Today, it seems to me that I was asking the wrong questions - or else I wasn’t asking the right ones. I was skeptical when my father told me that my grandfather had protected a Jewish employee at his haulage company. At school, such stories had been revealed to be self-serving lies all too often. So I sought out the man in question, and he confirmed that my grandfather really had saved him by letting him work for the family firm. The consul had been a really fine man, the employee told me shortly before he died, and he always thought of him with gratitude and respect.

Wrong question, right answer? Because what did I really know about my grandfather?

Ever since the NSDAP membership cards were handed over by the US to the German Federal Archives in 1994, more and more well-known names have surfaced. Politician and artists, academics and journalists, leftist liberals and conservatives. They all have just one thing in common: they grew up in the Third Reich and after the war went on to become prominent intellectuals and leading figures of the young Federal Republic of Germany.

It is a new curtain-closing debate, one in which a younger generation is expected to brush aside even the slightest of doubts on the biographical integrity of their role models, and to accept a black-and-white past peopled with evil Nazis and the good founders of the Federal Republic who got rid of them. The idea that even fractured biographies have the potential to be instructive and exemplary simply doesn’t fit into the dogma of these later-born high priests of Vergangenheitsbewältigung - the struggle to come to terms with Germany’s past. I know the truth about good and bad Nazis isn’t that easy, because my grandfather was one - or the other.

Post-War Lies: Germany and Hitler’s Long Shadow by Malte Herwig (translated by Jamie Lee Searle and Shaun Whiteside) is published by Scribe UK.