World War II is the deadliest war of the 20th century, with an estimated 20MM combined casualties. It’s important to note that some estimates are as high as 35MM casualties. WWII was fought between 1939-1945.
World War I is the second deadliest war of the 20th century. Waged between 1914 and 1917, WWI claimed over 8.5MM lives.
The Korean War was fought from 1950-1953 and resulted in over 1.7 million casualties.
The Chinese Civil War occurred from 1945-1949 and claimed over 1,200,000 lives.
The Vietnam War, occuring between 1965 and 1973 claimed over 1.2 million lives.
The Iraq-Iran War, 1980-1988, claimed over one million lives (greater than the entire population of Austin, Texas)
The Russian Civil war was fought from 1918-1921 and resulted in over 1,000,000 lives lost.
The Burundi-Rwanda Civil War is the most recent war on this list, occurring between 1993 and 2000. This war also claimed an estimated 1 million lives
The Chinese Civil war claimed about 400,000 lives between the years of 1927-1937
The French Indochina war resulted in 400,000 deaths between the years of 1945 and 1954