When Ukrainian historian Stanislav Kulchytskyi began poring through thousands of declassified secret police files in Kiev, he felt as if his eyes had finally been opened.

The files contained reports, letters, telegrams and directives all relating to the famine in 1932 and 1933 that killed more than three million Ukrainians. Many historians like Prof. Kulchytskyi had long concluded that the famine was a man-made disaster and genocide, imposed by Soviet leader Joseph Stalin to squash growing Ukrainian nationalism. But it was only after the Ukrainian government recently opened up public archives and declassified hundreds of thousands of documents that researchers have started to get first-hand accounts of what really happened. And there is much more to come. Thousands of new documents are coming to light almost daily, offering more insights into the tragedy.

“It is a little bit like a vase that is broken into small pieces and we are trying to piece the various pieces together to create the full vase,” Prof. Kulchytskyi said through a translator from Winnipeg, where he is starting a Canadian tour to discuss the famine and his findings. “We are trying to understand, what was the original thinking that Stalin had?”...