http://247wallst.com/special-report/2015/05/21/the-most-expensive-wars-in-u-s-history/
Declared an official holiday in 1971, Memorial Day honors those who have given their lives in service to the United States. While the human toll is always great, wars also cost treasure as well as blood. Many factors can affect the cost of waging war. Using a report from the Congressional Research Service, 24/7 Wall St. reviewed the most expensive wars in U.S. history. While the Mexican-American War cost just $2.4 billion, or 1.4% of GDP in 1847, spending on World War II accounted for nearly 36% of GDP in 1945, or $4.1 trillion. These are the most expensive wars in U.S. history.
Many early wars in U.S. history resulted in the acquisition of land. The Mexican-American War in the 1840s yielded much of the territory that makes up the present-day Southwest. Similarly, the Spanish-American War prior to the start of the 20th Century ended with U.S. control of Guam, Puerto Rico, and the Philippines.
In every conflict before World War II, nearly all of the country’s defense budget was spent on direct conflict — classified as wartime spending. For example, the U.S. spent 1.1% of GDP in 1899 to fight the Spanish-American War, and just 1.5% of GDP was spent on total defense spending.