By the time Soviet troops arrived at Auschwitz on Jan. 27, 1945, the vast Nazi death-and-slave-labor complex in southern Poland was nearly empty. Ten days earlier, the SS had evacuated the remaining 67,000 inmates, sending them on brutal forced marches westward, to concentration camps inside Germany and Austria. Thousands died along the way; any prisoner who collapsed or couldn’t keep pace was shot. Those who survived the death march out of Auschwitz — including Markus Jakubovic, a Jewish teen from Czechoslovakia who later became my father — faced months of additional terror, starvation, violence and slavery. For them, liberation would not come until after Germany’s surrender in May.

The few thousand prisoners still in Auschwitz when the Soviets entered had been left behind because they were sick or hiding. In Primo Levi’s description, they were “ragged, decrepit, skeleton-like patients,” dragging themselves “on the frozen soil like an invasion of worms.” Many were emaciated and covered in excrement; some were children who had been subjected to sickening medical experiments.

The soldiers who liberated Auschwitz 75 years ago this week they had come upon the largest mass-murder site in human history. Even now, the staggering immensity of the evil committed there is more than a normal mind can fathom. Between 1941 and 1945, at least 1.1 million men, women, children, and babies were killed in Auschwitz, 90 percent of them Jews. That is more than the combined US death toll in every war and military operation of the last 155 years. Tally every American killed in the Spanish American War, World Wars I and II, the Korean War, Vietnam, the Gulf War, Somalia, Iraq, Afghanistan — and the total comes to less than the number of Jews murdered in Auschwitz.

And Auschwitz accounted for but a fraction of the Holocaust.

Even as Germany fought the Allied armies on multiple fronts, it never relaxed its parallel war to annihilate the Jews of Europe. To that end it established tens of thousands of concentration camps, killing fields, forced labor sites, and ghettoes. But nowhere was the war against the Jews waged with greater, more terrifying totality than Auschwitz.

From the time the packed deportation trains pulled to a stop and the doors were thrown open, it took no more than an hour for everyone aboard to be robbed, stripped, and herded toward the gas chambers. Except for a small number of prisoners who were “selected,” tattooed, and diverted to slave labor, all the others — nearly always dazed, exhausted, frightened, and agonizingly thirsty after having traveled for days in a cramped, windowless boxcar — were hustled with industrial briskness to a painful mass death. When my father and his family were transported to Auschwitz in the late spring of 1944, the SS was gassing Jews to death at the rate of 6,000 per day — 180,000 per month. My father was selected for labor. His parents and young siblings were dead before he even knew what was happening.

Western leaders, however, knew only too well what was happening. As early as November 1941, Winston Churchill denounced in a radio broadcast “the unspeakable evils” being inflicted on Jews “by Hitler and his vile regime.” In 1942, American diplomats first learned — and notified Washington — of a Nazi plan to exterminate millions of Jews with poison gas. In January 1944, US Treasury Secretary Henry Morgenthau hand-delivered a memo to President Roosevelt that described in detail the refusal of State Department officials to cooperate with efforts to rescue European Jews. Drastic action must be taken immediately “to prevent the complete extermination of Jews in German-controlled Europe,” FDR was told. Otherwise, “this government will have to share for all time responsibility for this extermination.”

Nothing was done. Pleas to bomb the Auschwitz gas chambers, or at least the railway lines leading to the killing facilities, were repeatedly rejected. American officials insisted that resources could not be diverted from the war effort — yet in the late spring and summer of 1944, Air Force bombers repeatedly struck industrial targets nearby. On a single morning in August, the historian David Wyman documented, “127 Flying Fortresses, escorted by 100 Mustang fighters, dropped 1,336 500-pound high-explosive bombs on the factory areas … less than five miles to the east of the gas chambers.”

Saving Jews was not a priority. And so the gassing of Jews at Auschwitz continued until November 1944.

In the years since World War II, the civilized world has repeatedly vowed that it would never again avert its gaze when tyrants commit mass atrocities. The Holocaust was the worst crime in history, but at least this good came from it: Decent people learned not to shrug at genocide.

Or did they?

“Never again will the world stand silent, never again will the world look the other way,” said Jimmy Carter in 1979, upon receiving the report of the special commission on the Holocaust chaired by Elie Wiesel. Yet for three years he and the world had looked the other way as Cambodia’s Khmer Rouge conducted an astonishingly brutal campaign of slaughter in which an estimated 2 million people — a third of Cambodia’s population — were shot, starved, tortured, and overworked to death.

At the dedication of the US Holocaust Memorial Museum in 1993, Bill Clinton declared that the Nazi genocide would remain “a sharp thorn” in the conscience of the United States, which was shamed by “the deaths of millions whom our nations did not, or would not, or could not save.” His fine words were put to the test barely 12 months later, when Hutus in Rwanda launched a genocide against that country’s Tutsi minority that wiped out 800,000 victims in about 100 days. The US government under Clinton knew what was happening, and refused to intervene.

In 2012, Barack Obama mourned “the atrocities we did not stop and the lives we did not save” during the Holocaust, but avowed that “preventing mass atrocities and genocide [was now] a core national security interest and a core moral responsibility of the United States.” On Obama’s watch, however, there would be no meaningful response to horrific violence unleashed by Syrian dictator Bashar al-Assad against the country’s civilians. As 500,000 people were murdered through bombings, chemical weapons, and starvation, America declined to halt the killing. Not even a plea from dozens of US diplomats could persuade Obama to act.

For 75 years, the searing memory of the Holocaust has been invoked with the reverential words: “Never Again.” And for 75 years, “never again” has become “yet again.” Not only in Cambodia and Rwanda and Syria, but also in Congo and Kurdistan, in East Timor and Darfur, in Bosnia and Guatemala, in the North Korean gulag and the concentration camps of Xinjiang.

Murderous ethnic cleansing is underway even now in Myanmar, where tens of thousands of Rohingya civilians have been butchered, burned, and raped to death by government forces aided by Budhist militias. The horrifying details have been widely reported. The blood of the victims cries out from the ground. But America and the West will do nothing to stop the monsters. “Never Again” is a fine sentiment for Holocaust memorials and presidential speeches and Hollywood films about the lessons of Auschwitz. But as a guide to action, it is useless.

(Jeff Jacoby is a columnist for The Boston Globe).