For decades, the attempts made by the United Nations War Crimes Commission (UNWCC) to prosecute the horrific crimes committed by Nazi Germany during World War II have been shrouded in secrecy. This week, tens of thousands of files that make up the commission’s archive will be opened to the public for the first time at the Wiener (pronounced “VEE-ner”) Library in London, a leading research destination for scholars of Nazi Germany, the Holocaust and genocide. The catalog will be available to search online. As reported in the Guardian, the opening of the archive promises to “rewrite crucial chapters of history” about the Holocaust and how the Allied Powers confronted it.

“The first takeaway is that it completely changes how we thought the Allies responded to the Holocaust,” Dan Plesch, a professor at the Centre for International Studies and Diplomacy at SOAS University of London told HISTORY. “We now know that part of the Allied governments in London…in fact brought criminal indictments against Hitler and his henchmen while the war was still going on.”

Plesch has been working with the UNWCC archive for a decade, and his new book, “Human Rights After Hitler: The Lost History of Prosecuting Axis War Crimes,” was published this week in concert with the archive’s opening in London. According to his research, the commission’s records showed that that Allied governments were aware as early as December 1942 that the Nazi regime had murdered at least 2 million Jews. In fact, the U.S. and U.K. governments made a public declaration about the slaughter at that time, nearly two-and-a-half years before the discovery of the first concentration camps.

Ben Barkow, director of the Wiener Library, also spoke to HISTORY about one of the other “attention-grabbing” revelations of the archive: the commission’s efforts to indict Adolf Hitler directly for his role in the massacres Nazi units carried out in Czechoslovakia.

“It was more a political gesture than a serious attempt to bring him to trial,” Barkow noted. “I believe that under the auspices of the war crimes commission you couldn’t indict a head of state. But it was nevertheless an important gesture”—and one that was not widely known about until now. The commission secretly handed down that indictment at a meeting in late 1944, after assembling hundreds of pages of evidence to support it.

As Barkow explained, the archive also reveals early efforts Allied governments made to treat rape and sexual violence as war crimes. “Most people think it started after the Rwandan genocide and the war in the former Yugoslavia,” he noted. “But in fact the Allies were working on that even before World War II was over.”

Beyond the commission’s work, why didn’t the Allied governments say or do more to stop the Nazis’ horrific crimes during the war? According to Plesch, anti-Semitic forces within the U.S. State Department at the time pushed back against the efforts of Herbert Pell, the U.S. envoy to the UNWCC. Later, the State Department would agree to the more high-profile trials of Nazi leaders at Nuremberg in 1945-46, which overshadowed the earlier commission’s wartime prosecutions.

In the late 1940s, the UNWCC was shut down and use of its records effectively suppressed, as West Germany became a key U.S. ally at the start of the Cold War. For some 70 years, the documents remained tightly guarded, closed to researchers unless they got the permission of both their own government and the UN secretary general.

During the Cold War, the crusading anti-communist Senator Joseph McCarthy lobbied to end the war crimes trials altogether, and many convicted Nazis were granted early release. “The political forces of Sen. McCarthy bring the whole thing to a screeching halt and smear the whole process,” Plesch said.

But as the newly opened UNWCC archive makes clear, such equivocation about the horrors of the Holocaust was as wrongheaded—and dangerous—then as it would be today. Current world leaders, Plesch argued, could learn from the commission’s example (and its dogged efforts to document and prosecute Nazi crimes during World War II) when faced with evidence of the war crimes being committed in the Middle East today.

As for historians, journalists and the general public, the post-war cover-up of the commission’s efforts offers a lesson of history we shouldn’t forget. “Once upon a time,” Plesch said, “There were people not only suppressing war crime trials but convincing the world they never even existed.”