OTTAWA — When an individual suspected of taking part in the Second World War murder of Jews in western Ukraine applied for admission to Canada in 1951, immigration officials did not follow up with potential witnesses who might have provided crucial details. In another case, a Slovak leader hoping to unite émigrés under his leadership was allowed to visit Canada repeatedly in the 1950s and '60s, despite a record of war crimes. In 1962, the RCMP learned that a Soviet trial of concentration camp guards in what is now called Belarus had named two people living in Canada as active...