World War II, by far. Besides the Jews killed by Hitler's Nazis (6 million or more), at least 20 mill. Russians were killed. But the big loser were Asians; at least 40 mill. Of course all who fought in the war lost people but the Russians and Asians payed the biggest toll, in numbers at least. We will never know exactly how many died!
In terms of the number of Americans killed in a war (declared or undeclared), the American Civil War wins by a large margin.
Recent statistical analysis (as record-keeping was horrible) of Demographics indicates that somewhere between 600,000 and 900,000 Americans died due to the Civil War (killed, death due to disease while serving, or died of wounds received during the war). A "best guess" of 750,000 from this study is roughly 20% higher than previous estimates.
This number is large due to both sides being Americans (as is the case in any Civil War), and with the American Civil War being considered by historians to be the first industrial war, the death toll was radically higher than any prior conflict. The means of destruction had significantly outpaced the methods of warfare, resulting in higher than usual casualties.
For comparison, the total number of (American) war dead due to WW2 was barely 1/2 the above total, at about 418,000.
If the question being asked is read as "which war, that the United States participated in, have the most total death on all sides?", then the answer is most certainly World War 2, with a total of between 63 and 78 million deaths (civilian and military), dwarfing any prior conflict in human history. However, the second largest war death total was the An Lushan Rebellion (in 753 A.D.) in China, which killed 33-35 million people, which resulted in a much larger percentage of the world population of the time being killed (14% vs 2% in WW2).
The Civil War had the most AMERICAN deaths
The Soviet Union (U.S.S.R.). By far. And the most civilian deaths by far.
Disease and neglect
The USSR.
The Russians .
The Civil War had the most AMERICAN deaths
the most casualties in a war was the American Civil War with 495,332 deaths and behind it is WW2 with 405,399 deaths.
russia had the most deaths in ww1
Disease.
The Soviet Union (U.S.S.R.). By far. And the most civilian deaths by far.
WW1 at the Battle of the Sommes.
Disease and neglect
WW II.
The USSR.
Besides the obvious injuries and deaths, most deaths were the result of infection, dysentery, diarrhoea, secondary infections, pneumonia, and TB.
Union Civil War CasualtiesCombat Deaths: Over 110,000Other Deaths*: Over 250,000Confederate Civil War CasualtiesCombat Deaths: Over 95,000Other Deaths*: Over 165,000(*Other Deaths include, among others: disease (by far the most common cause of death), accidents, drowning, heat stroke, suicide, murder, execution.)
dresden