Original Cost: $288 billion
Inflation Adjusted Cost: $3.6 trillion
Just two decades after the War to End All Wars, the deadliest war in history began. After the attack on Pearl Harbor on December 7, 1941, the US formally joined forces with the Allies and declared war. WWII was the most costly endeavor this country ever has been through in terms of dollars and in terms of lives.
Note: CNBC based its computations for this "running tally" on government data and news releases, as well as estimates from a variety of sources.
References include US National Archive, US Dept of Defense, US Bureau of Reclamation, Library of Congress, NASA, Panama Canal Authority, FDIC, Brittanica, WSJ, Time, CNN.com, and a number of non-news Websites.