1941 followed by 1942
1941 (MCMXLI) was a common year starting on Wednesday of the Gregorian calendar, the 1941st year of the Common Era (CE) and Anno Domini (AD) designations, the 941st  year of the 2nd millennium, the 41st  year of the 20th century, and the  2nd   year of the 1940s decade.  
The Correlates of War project estimates this to be the deadliest year in human history in these terms of conflict deaths, placing the death toll at 3.49 million. However, the Uppsala Conflict Data Program estimates that the subsequent year, 1942, was the deadliest such year. Death toll estimates for both 1941 and 1942 range from 2.28 to 7.71 million each.