After WWI, Before WWII
In the famous words of Warren G. Harding, president from 1921 to 1923, the United States after WWI wanted a return to normalcy. This meant stepping back from so much engagement with Europe.
This did not mean that the US did not engage with the world altogether: the US still had colonies overseas, wanted increased trade with the rest of the world, and sought to limit the possibility of future wars through mediation and treaties.
For example, the Washington Conference of 1921 tried to stop naval arms races by establishing a ratio of battleships with the US and UK at the top, followed by Japan and then France and Italy (The US’ secret agenda was also to stop growing Japanese naval power in the Pacific). The Kellogg-Briand Pact (1928) was a promise by countries never to resort to war, but since it had no enforcement provisions, it was largely useless.
Finally, in a bit of foreshadowing, the US was a crucial part of setting up the Dawes Plan where US banks made loans to Germany, who then paid their reparations (penalty payments for WWI) to the UK and France, who then repaid American banks for WWI loans, and then the whole cycle repeated itself. This would work well until the 1930s and the Great Depression, of course.
As the Great Depression worsened and took hold in other countries, some of those countries turned to radical leaders who embraced nationalism, militarism, and expansion as a cure for their countries’ economic woes.
Germany elected the fascist leader Hitler, Italy had Mussolini, the Soviet Union (USSR) had Stalin, and Japan had Tojo. All four leaders were examples of totalitarianism where the state/government took over all aspects of life, and all began to expand their territories in the 1930s.
While the United States was nervous about the rise of these dictators and their militaries, most continued to support isolationism. This was true even through the late 1930s when Japan invaded China and Germany invaded Poland to start World War Two (WWII).
Good Neighbor Policy
Roosevelt promised a “policy of the good neighbor” toward other nations of the Western Hemisphere. The US delegation at the 7th Pan-American Conference in Uruguay in 1933, pledged never again to intervene in the internal affairs of a Latin American country.
FDR pledged to submit future disputes to arbitration and also warned that if a European power, such as Germany, attempted “to commit acts of aggression against us” it would find “a hemisphere wholly prepared to consult together for our personal safety and our mutual good.”
FDR suspected that war would come to the US too, so he began to pressure the US to prepare, even amidst public opposition to US involvement. He thus moved slowly and in a piece-by-piece fashion. The Neutrality Acts passed in the 1930s made it difficult for the US to trade with nations involved in the war to avoid similar economic entanglements to WWI.
🎥 Watch: AP US History - Interwar Period
US Assistance to the Allies
FDR thus got the US involved piece by piece: first the US used Cash and Carry where the British would pay for war material in cash (no loans to draw the US in!) and carry it across the Atlantic themselves (no German U-boat problems this time!).
When the British ran out of cash, the US traded old Destroyers for Bases that the British had in the Caribbean. Finally there was the Lend Lease Act where the British borrowed money and material from the US (this would eventually include China and the USSR as well). FDR also instituted the Selective Service Act, the first peacetime draft in US history in 1940.
All these actions were opposed by Americans allied with the Nazi party and those who wanted traditional US isolationism. The most famous of these were Charles Lindbergh and the organization America First.
It finally took the Japanese attack on the US naval base at Pearl Harbor
(Dec. 7, 1941) to push the US public fully into the war, leading the US Congress to officially declare war against Japan (and eventually Germany and Italy too).