Causes of US Involvement in World War II

Following World War I, the United States adopted an isolationist stance. Starting in 1935, 

Congress even passed various neutrality acts to enforce the will against foreign 

entanglement. But by December of 1941, President Roosevelt’s formal declaration of war 

made this legislation irrelevant. 

Although America attempted isolationism, European and Asian affairs brought global 

tension that eventually hit the country’s traditional allies. An aim of World War I had 

been “to make the world safe for democracy”, but democracy in the 1930s was 

increasingly endangered. The roots of World War II lay in the totalitarian leaders of Asia 

and Europe and their agendas for expansion. 

Totalitarianism emerged in the Soviet Union, Italy, Spain, and Germany. The fascist 

leaders had expansionist goals and soon crushed neighboring societies. Italy invaded 

Ethiopia and established Italian East Africa. Meanwhile, Japan invaded Manchuria, 

seized Chinese land, and occupied French possessions in Southeast Asia.

In 1938 Europe, the war officially began when Germany’s Adolf Hitler invaded Austria 

and took Czechoslovakia’s Sudetenland, which was home to 3.5 million ethnic Germans. 

Hitler claimed he was only “restoring rightful boundaries”, since Germany had lost 

territory in World War I. But Hitler had ideas of widespread domination. In 1939 he and 

Mussolini created the Rome-Berlin Axis alliance, a military agreement designed to last 

ten years. Japan entered the pact later that year. Hitler had the confidence to invade 

Poland in 1939. Poland’s allies, England and France, therefore declared war on Germany. 

America’s traditional allies were at war.

Initially, President Franklin Roosevelt limited his aid to arms sales, which were restricted 

in a neutrality act. But Hitler’s invasions continued. He took Denmark, Norway, and 

Holland, and the Belgian king surrendered his army shortly thereafter. And in June of 

1940, France succumbed to Nazi forces. The Axis alliance now dominated Europe from 

the North Cape of Africa to the Pyrenees. Great Britain’s Winston Churchill vowed to 

continue the battle for democracy.

Churchill soon needed military aid, and Roosevelt declared that the United States must 

become “the great arsenal of democracy”. By 1941, he officially ended the country’s 

isolationist stance by passing the Lend Lease Act, which lifted restrictions on supporting 

foreign troops with defense gear; the Act first appropriated $7 billion to lend or lease 

supplies to any countries the president designated. President Roosevelt also started to call 

US National Guard members to war training.

Next, the Americans built a base in Greenland. Then, stationed aboard warships near 

Newfoundland, Roosevelt and Churchill issued the Atlantic Charter in June of 1941. 

Although the US had not officially entered the war, the Atlantic Charter presented the 

two countries’ goals for a war against fascism. It included their disinterest in acquiring 

new territories through the war. Shortly thereafter, the US became involved in the years-

long Battle of the Atlantic.

The United States officially entered World War II in December of 1941. Japanese 

military leaders, led by Admiral Isoroku Yamamoto, attacked a US naval base in Pearl 

Harbor, Hawaii. The Japanese aimed to destroy the US fleet docked in the Pacific, thus 

leaving the Japanese free to pursue oil mines in the region. A series of aerial attacks by 

361 airplanes succeeded in compromising eight important warships. The air attacks also 

killed more than 2,300 people. The following day, President Roosevelt asked Congress 

for a declaration of war against Japan. Congress obliged. By the time of this official 

declaration, there were battles to fight on many fronts, but “Remember Pearl Harbor!” 

became a rally cry for the war.