The American Civil War (1861-1865). In it's most heated year the Civil War most likely cost over 300 fatalities per 100,000 citizens a year. This is not counting those who died from disease. Since it is also one of the only wars to have taken place on US soil, it can also be safely assumed to be the costliest in terms of monetary damage as well.
The next most deadly conflict was WW2 which cost the US about 10-140/per 100,000 citizens each year. In descending order, other costly conflicts were...
American Revolution
War Of 1812
Korean Conflict
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The most destructive tornado in U.S. history was the Joplin, Missouri tornado of May 22, 2011. The cost of damage was $2.8 billion.
The French and Indian War was the longest US war before 1850.The longest US war in history was the Vietnam War.
The Civil War (Over 650,000 deaths)
We can not answer your question because you have not specified which country's civil war you are asking about. Most countries have had a civil war at some time in their history. Please make your questions clear and specific if you want us to answer them for you.
slavery civil war . . .