Final answer:
The United States primarily financed the war through the sale of war bonds, supplemented by income tax revenue after the Sixteenth Amendment's passage.
Step-by-step explanation:
The United States raised most of the funds needed to pay for the war primarily through the sale of war bonds. As part of its financing strategy, Congress authorized the sale of bonds to the American public. This was central to their efforts, as it provided a substantial amount of money required for wartime expenditures. Besides war bonds, additional revenue was raised through federal income tax, which became possible following the passage of the Sixteenth Amendment in 1913. While taxes did contribute, they accounted for a little over 16 percent of war costs. The use of Liberty Bonds, in particular, was significant, raising $23 billion, as patriotic citizens were encouraged to “do their part” to support the war effort and help bring troops home.