What on earth were the German people thinking during World War II?

How could they support such a dreadful war of aggression? How could they tolerate – or actively support – the extermination of Jews and others who failed to measure up on the Aryan scale of racial superiority?

And why did so many support the Nazi war effort to the end, despite the devastation it brought and overwhelming evidence that the German military could not prevail?

Historian Nicholas Stargardt should be applauded for answering those questions in “The German War: A Nation Under Arms, 1939-1945.” He’s done much to fill in a gap in the historical record of World War II. What motivated ordinary Germans, what they knew – and when – is now less of a mystery.

Figuring that out surely was a difficult task. Faulty memories and historical amnesia were epidemic in Germany after World War II.

Much of Stargardt’s research relies heavily on a paper trail of personal letters, police and court records, and propaganda ministry assessments of morale. His portrait of ordinary Germans during World War II is nuanced, revelatory, at times empathetic, but also damning.

On the eve of the war, many Germans still harbored feelings of victimization. They saw World War I’s outcome as the result of defeatism and betrayal. Their resentment smoldered over harsh economic sanctions imposed by the victors. They worried about unresolved geopolitical issues, especially to the East.

Germans from many walks of life, even some Jewish nationalists at first, welcomed the Nazis’ “national revolution.”

Even so, Hitler’s decision to go to war in 1939 was “deeply unpopular” in Germany. Yet Germans also viewed the conflict – then and to the bitter end – as a war of national defense rather than as a war of German aggression.

The war’s popularity waxed and waned. Stargardt writes that German soldiers were almost giddy during the invasion of France, and trepidation gave way to euphoria at home.

Stargardt methodically tears down any pretense that Germans weren’t aware of war atrocities and the Holocaust. And that knowledge came early in the war.

“Many of the soldiers who witnessed such events across Poland took rolls of photographs, which they sent home to be developed and printed,” Stargardt writes. “In this way, a visual record passed through the hands of parents, wives and photographic assistants before being returned to the ‘execution tourists’ in Poland.”

Later in the war, Soviet soldiers “found thousands of images of killing sites in the uniform pockets of German prisoners and dead, kept next to pictures of their fiancées, wives and children.”

Germans began opining as early as 1943 that Allied bombing raids on their cities were retaliation for the murder of Jews.

The nation’s clergy, who arguably could have been a strong moral compass that steered their congregations away from the Nazis’ most banal policies, did little. Many Catholic and Protestant intellectuals hoped that the Nazis would lead a spiritual revival, Stargardt writes. A few clergy did speak out against the regime, but not nearly often enough.

Catholics and Protestants alike failed to stop systematic murder of patients in Germany’s psychiatric asylums. Most turned a blind eye to the fate of the Jews. Some Protestant leaders even asserted in 1941 that “racially Jewish Christians have no place and no right to be in the Church.”

What might have happened if Catholics and Protestants had raised their voices more forcefully is worthy of conjecture. A Nazi attempt in 1942 to remove crucifixes from schools in Bavaria provoked a strong response. Ultimately, Stargardt writes, the Nazi Party and the SS backed down and “did not dare initiate an open conflict with the Church again during the war.”

Even Nazi Germany’s reputation as a police state takes several hits in Stargardt’s well-researched history.

“The Gestapo might enjoy its omnipresent, omniscient and omnipotent reputation, but its totalitarian aspirations were curtailed by staff shortages, which became worse during the war,” he writes.

As the fortunes of war took a turn for the worse, “party officials were so often abused and threatened in public, especially in cities which had recently been bombed, that in the late summer of 1943 many stopped wearing their uniforms and party badges in public.”

But Germans continued to fight or support the war, despite growing misgivings. The illusion that secret super weapons could turn the tide may have contributed to that. And the fear of retribution for German conduct during the war certainly loomed large.

Even in defeat, with the enormity of what Germany had done all too apparent, unsettling attitudes persisted.

Shortly after the war ended, Allied investigators were told that wartime bombing was seen by many Germans as “the revenge of World Jewry.” Postwar polls in 1945 and 1946 showed that almost half the population saw National Socialism as “a good idea carried out badly.”

And as the Cold War heated up, “old professional elites were also welcomed back into the West German state.”

Today, there’s little tolerance for the attitudes expressed by Germans during the war years. The German response to the Syrian refugee crisis would have been incomprehensible to the war generation.

So, in addition to writing an outstanding history, perhaps Stargardt also has created a baseline that shows just how much Germans, and Germany, have changed.