Under America I suppose you interpret the U.S. War bonds were a necessity in World War 1 when the U.S. needed financial background to support it's army and issued the Liberty Bonds. You can guess from the name the bond scheme was designed to rely on the patriotic mindset of the U.S. citizens. In the interwar period Baby Bonds became increasingly popular as one did not need much capital to invest and the interest rate were very good for the time. By 1940 the victories of Nazi Germany convinced U.S. government officials that they will need to prepare the country financially. British economist John Keynes proposed to increase taxes and initiate a mandatory savings program to keep inflation low and spendings high. He had many supporters in the White House, although the Secretary of the Treasury Henry Morgenthau, Jr. however preferred a voluntary loan system. Instead of following the ideas of Keynes series of baby bonds were issued as "defense bond". If one couldn't buy a whole bond it was possible to buy stamps, which combined would equal a bond. After the attack on Pearl Harbor interest rates rose and the bonds were renamed as War bonds.

War doesn't run on patriotism. Factories, airfields, weapons, training, munition and transports all cost money. To coordinate an effort to prepare the army for an upcoming conflict the need for liquid assets increases. Instead of enforcing their own population to pay the governments increased expenses the U.S. government has chosen a voluntary loan system. This ensured that the general public was not enforced to pay for a war they did not necessarily support (there were many who supported a "passive U.S." until Pearl Harbor), but those who wished could make an investment with quite acceptable terms. By the end of the war the U.S. have spend approximately 240 billion $, from which 40% were collected via taxes, the rest of this amount was procured via bonds.

War bonds were to limit inflation by reducing the money supply.