World War II QUIZ: The Deadliest War In History History 12/18/23 Which country had the largest number of casualties? 0% Retry Correct Incorrect The answer is Russia. They lost over 21 million lives in WWII. Next Keith Hamshere/Getty Images Keith Hamshere/Getty Images Which country had the largest number of casualties? Japan Russia America United Kingdom 0% Scroll down to continue on! Next