Nazi aggression and the Blitzkrieg's military imbalance
Sources:
Evan Mawdsley, Thunder in the East: The Nazi-Soviet War, 1941-1945 (Hodder Arnold, 23 Feb. 2007)
Donald J. Goodspeed, The German Wars (Random House Value Publishing, 3 April 1985)
Greig Watson, “Operation Gomorrah: Firestorm created 'Germany's Nagasaki'”, BBC News, 2 August 2018
Chris Bellamy, Absolute War: Soviet Russia in the Second World War (Pan; Main Market edition, 21 Aug. 2009)
Luiz Alberto Moniz Bandeira, The World Disorder: US Hegemony, Proxy Wars, Terrorism and Humanitarian Catastrophes (Springer; 1st ed. 2019 edition, 4 Feb. 2019)
Robert Service, Stalin: A Biography (Pan; Reprints edition, 16 April 2010)
David C. Gompert, Hans Binnendijk, Bonny Lin, Blinders, Blunders, and Wars: What America and China Can Learn, Chapter 7 Hitler's Decision to Invade the USSR, 2014, Publisher: Rand Corporation, p. 4 of 12, Jstor