We must never forget.

It is the creed of the survivors who braved unspeakable horrors at the hands of the Nazis. The brave Allied soldiers who battled to free them share it, too.

But the reality is, as time moves forward, fewer and fewer firsthand voices remain. As they go, so go the memories.

It is for this reason that author Michael Hirsh sought to document for posterity the stories of the American heroes who swept across Europe in the waning days of World War II in order to free thousands upon thousands of Jews and others marked for death.

“The Liberators: America’s Witnesses to the Holocaust” is Hirsh’s homage to these GIs, and it is a fine and necessary addition to the lexicon of Holocaust literature.

A compilation of 150 interviews with aged veterans, the book chronicles — in paragraphs and sometimes pages of the soldiers’ own words — the Americans’ inexorable march from the beaches of France in 1945 through Axis-controlled Germany and Austria to liberate the brutal Nazi death camps.

But the book is not simply an aloof account the liberation. It is a powerful and emotional telling of the trials these soldiers faced.

As testament to the horrendous cruelty of Adolf Hitler’s death machine, war-hardened soldiers, fresh from the battlefields, were overwhelmed by the breadth of inhumanity they witnessed in the concentration camps. It is a testament to the fragility of the human psyche that these sights haunted the soldiers for the rest of their lives.

As put by Werner Ellman from the 11th Armored Division, which liberated Mauthausen in Austria: “We had seen killing. For Christ’s sake, that was nothing new to us, but this was too hard to handle.”

The putrid stench of burning human flesh from crematoriums that permeated for miles. The walking skeletons in striped pajamas, begging for food or propping themselves up on chain-link fences for lack of strength. The rooms filled with thousands of pairs of shoes. Lampshades made from human skin. And the stacks of decaying bodies that so many soldiers described as piled “like cordwood.”

All these things are documented in detail and often accompanied by pictures because this is exactly what the liberators saw. And if the stories seem repetitive, it’s because the soldiers’ experiences were often analogous from one concentration camp to the next.

But reactions varied. Some soldiers internalized the scene. “It was business,” John P. Marcinek, who along with the 104th Infantry Division freed Nordhausen, told Hirsh.

Others followed Gen. George Patton and vomited on the spot. Others still simply broke down crying or lapsed into prayer.

All soldiers, though, were met with a hero’s welcome from the woeful prisoners.

“Their kindness and their thoughtfulness gave us back our belief in the human race,” wrote one-time Salzwedel internee Lea Fuchs-Chayen in a public letter thanking the saviors.

Some soldiers, so enraged by the atrocious sights, killed any remaining German guards on the spot. Other didn’t have to; the prisoners beat them to it. Some commanders ordered their troops to round up nearby villagers — German citizens who claimed to know nothing of the camps, despite the wretched smell that seeped into their towns — to make them bury the dead.

Hirsh is sympathetic to his interviewees, but he does not censor them. It was war.

But what is most striking in these stories is just how deeply affected each and every soldier was by the scenes they witnessed in 1945. Now in their 80s and 90s, the veterans’ lives have been marked by nightmares, post-traumatic stress disorder, denial and rage.

Some repressed the memories and not until a half-century later did they verbalize their experiences. Don Timmer, who helped liberate Ohrdruf with the 89th Infantry Division, for instance, only spoke up after a Loudonville, Ohio, school board member challenged the school’s teaching of the Holocaust, saying it was exaggerated.

Fredrick Krenkler, who liberated Dachau with the 42nd Infantry Division, said years of isolation and broken relationships are the toll the war took on him. Stanley Friedenberg, who was with the Counterintelligence Corps, lost his faith in God entirely.

This book is not an easy read. It is extremely distressing; the accounts are heart-wrenching. But it could not have been harder to read than it was to write. And that, still, must have been infinitely easier than experiencing the horror of the Holocaust firsthand. What we must not forget, the soldiers cannot forget.

As Yom HaShoah (Holocaust Remembrance Day) approaches on April 11, Hirsh’s work is a fine way to commemorate the Holocaust and the courageous Americans who helped bring it to an end.