What was the most costly war for the us

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. May 24, 2015