5. World War I: $334 billion
In 1917 the U.S. joined WWI, a year before the four-year conflict ended. The battle was fought between two groups of regions/countries.
But the war began in 1914, a little early before U.S. decided to join and it ended one years after America joined Allies.
Before that Europe was divided into two groups consisting of the Allied Powers of Britain, France and others. On the other hand there were the Russian Empire, Italy, Japan and the United States.
The Central Powers included Germany, Austria-Hungary, the Ottoman Empire and Bulgaria.
As one of the largest wars ever, more than 70 million people fought in World War I and that reflected well into the costs.
