Though the USA was already the world’s most powerful economy at the start of the 20th century, American public opinion was mostly isolationist, i.e. it wanted in a sense to ignore the world. This was the case up to the late-30s, the only overseas intervention being the sending of troops to fight in the trenches of WW1. President Woodrow Wilson asked Congress to get the USA involved in the "war to end all wars", that would "make the world safe for democracy", and so Congress declared war on Germany in April 1917. In July, General Pershing led the US expeditionary force to France and the US intervention hastened the victory of the Entente Powers. President Wilson, hugely popular in Europe, was seen as the peace-maker at the Paris Peace Conference in 1919, wanting "no annexations, no contributions, no punitive damages..."
Wilson had given a speech to Congress in January 1918 called the Fourteen Points in which he explained the US’s decision to get involved in the conflict, namely because violation of international law had been committed (German U-boot submarines had sunk ships in which there had been Americans and US merchandise, most famously the Lusitania) and therefore Germany had to be punished. Wilson also explained that justice had to be upheld all over the world and that world peace was the ultimate aim of his Fourteen Points.
Wilson received a tumultuous welcome in Paris. Next to him is French President Raymond Poincaré.
Library of Congress photo (December 14th 1918)
Only point (14) of Wilson’s program was in the end included in the Versailles Treaty (1919). The victorious European leaders were not interested in a just peace : they wanted revenge and to make the defeated countries pay for the war (and Wilson was not, contrary to expectations, a good negotiator)... The US Congress actually refused to ratify the Versailles Treaty and did not want the USA to become a member of the League of Nations.
Warren Harding, who succeeded Wilson, was elected President in 1920 because he advocated a “return to normalcy” i.e. isolationism. Over 100,000 US soldiers died because of the War; this and the horror of the fighting convinced people to vote Republican. The war had not been popular before the country got involved in the conflict (not just among many Republicans but also among workers and socialist activists) and it was not better considered during or after it. Indeed, Wilson had to employ conscription to raise enough soldiers, and make widespread use of propaganda to convince the population that the country's military involvement was justified (the 1917 Espionage Act and the 1918 Sedition Act were a means to quell opposition).
Wilson got his country involved in the war because he probably had no choice (Americans and American interests had been attacked), and he had to justify his interventionist policy by saying that it was to create a world of greater peace (compatible with trade) in which negotiation, cooperation, respect of common rules prevailed… The idealism of his multilateral diplomacy was applauded but largely rejected (apart from the setting up of the League of Nations). America did not, as Harding described it, “submergence in internationality”. For the next twenty years (the interbellum: 1919-1939) the USA went back to its isolationism on the political level.
The US’s isolationism even had an effect on immigration: the Johnson-Reed Act was passed in 1924 and restricted the number of people coming into the country. In 1930, 10% of Americans were foreign-born (15 million people); in 1970 it was 5% (10 million people). The Johnson-Reed Act was repealed in 1965. Today, 12% of the population is foreign-born (22 million people).
Though isolationist politically, the US continued to expand its business interests abroad throughout the 1920s, especially in South America (Foreign Direct Investment there went from about 150 million dollars in 1910 to about 2,500 million dollars in 1928).
During the 1930s Great Depression, the US continued to be isolationist because of the crisis that started with the 1929 Wall Street Crash: America was in a sense too busy solving its domestic problems to worry about the rest of the world. Unemployment rose dramatically and the American (economic) model was called into question by Americans themselves. The Hawley-Smoot Tariff launched in 1930 increased import tariffs on thousands of products from abroad. This protectionism by the USA meant that trade with the rest of the world slowed considerably and exacerbated the world-wide crisis.
Roosevelt launched his New Deal to try and solve the crisis at home: State intervention in banking, finance, trade, large-scale public works, and help for the poor. The New Deal gave renewed hope to Americans. Roosevelt however had little concern for what was happening abroad (at the London Economic Conference of 1933, which tried to find a multilateral solution to the monetary and trade crisis, the USA placed its national interests first, especially as the countries of the world were mostly indebted to American banks).
It is the Japanese attack on Pearl Harbor on 7th December 1941 (2400 servicemen killed) that made the USA get involved militarily in WW2. Up to then it had supported the UK and the USSR materially (via the Lend-Lease program of 1941 to “…Further Promote the Defence of the USA”). The USA realized it had to act with the Allies against the Axis Powers to protect its national interests (and it was in its national interest too to boost the economy through the manufacture of arms…).
FDR and Churchill in August 1941 meeting to discuss the Atlantic Charter
The USA led the Allies to victory, having the most powerful army, and the material means to win. The USA had a strong will to win: America and the Allies were the forces of “good” and they were fighting for freedom and democracy against evil (the fascist regimes). In August 1941, the USA had justified its potential involvement in the fighting in the Atlantic Charter: it would be to further economic collaboration, to promote the self-determination of countries, etc.
Celebrating the Allied victory in WW2
WW2 made Americans proud of their country, united them, and gave them back their self-confidence and economic strength. By the end of the conflict, the USA had become a superpower, with a very strong world-wide influence on political, economic, military and cultural matters.
The assets of the USA in 1945 were:
- It could lend money
- The dollar became the standard currency for the exchange rate (cf. Bretton Woods)
- Its industrial capacity
- Its military capacity
- Until 1949, it was the only country with the atomic bomb
- Its “American way of life” serving as an economic, political and socio-cultural model