Why did the United States enter World War II?

The United States entered World War II after the Japanese Navy attacked the U.S. naval base at Pearl Harbor on December 7, 1941. This was the culmination of a long process in which Japanese imperial ambitions in the Pacific and German aggressions in Europe threatened to drag a reluctant nation into war. In the 1930s, the United States responded to German and Japanese aggressions by maintaining its own neutrality. But President Franklin Roosevelt was always skeptical that the United States could remain aloof from the affairs of the rest of the world. In 1939, the United States began to provide military assistance to Great Britain, by then at war with Germany, on a "cash and carry" basis. Later, after Hitler overran Europe, Congress approved a "lend-lease" plan that involved massive loans of cash and war materiel to the British, the Soviet Union, and China, all of which were at war with the Axis Powers. But still the United States remained out of World War II. This changed in 1941 when, in response to the Japanese invasion of Indochina, Roosevelt ordered the cessation of oil exports to Japan, and froze Japanese assets. The Japanese then formulated a plan to attack the American naval forces based at Pearl Harbor.

Why did America enter WWII?

One can easily point to the Japanese attack on Pearl Harbor as being the reason why the United States entered World War II. But while this is certainly true, the answer regarding why the country entered the war is more nuanced than simply thanks to the occurrences of a single event.

Even as late as 1940, U.S. President Theodore Roosevelt had told the American people that their "boys are not going to be sent into any foreign wars." However, British Prime Minister Winston Churchill was persistent in asking for U.S. assistance and participation in World War II. Some believe that Roosevelt was taking steps toward bringing the United States into the war even before the bombing at Pearl Harbor.

After the attack, only one congressman opposed the declaration of war. If the attack hadn't been enough to provoke the United States, then Germany's declaration of war against the United States just four days later certainly would have been. 

How and why did the United States enter World War II?

We were also quite involved on the Atlantic front by the time the attack on Pearl Harbor occurred.  We had been giving large quantities of weapons and other aid to Great Britain through the Cash and Carry and Lend-Lease programs.  To a lesser extent, we were also helping France and even the Soviet Union with such aid.

Fifty American destroyers were given to the British for convoy duty and American ships themselves started patrolling months before the US formally entered the war.  So we had been moving in the direction of war for quite some time, it just wasn't politically possible to do so until the Japanese attacked.

I lost two great-uncles when the Japanese bombed Pearl Harbor. George Stanley Miller and Jesse Zimmer Miller are still interred on the USS Arizona. Consequently, I feel a great sense of patriotism about the US's involvement in WWII. However, considering the atrocities Hitler was engaging, I think the US should have stepped up sooner.

The Japanese had previously been insulted when the United States had excluded people of Asian ancestry from immigration into the U.S. Then, when the United States stopped shipping oil and scrap metal to the Japanese, the belief of the Japanese government (which was belligerent to the core) was that war with the U.S. was inevitable. Admiral Isoroku Yamamoto had advised the High Command that the U.S. did not have a warrior image, and did not have the stomach for a long war. He believed that if the Pacific Fleet were eliminated, there would be ample opportunity to make peace on favorable terms before the Atlantic Fleet could be deployed to the Pacific. He was wrong, of course.

Pearl Harbor was, of course, the immediate cause of the US involvement, but there were less immediate causes that were very important.  Specifically, it was American efforts to prevent Japan from getting an empire that helped cause the Pearl Harbor attack.

The US did not want Japan to take the Asian empire that Japan felt it deserved.  This led the US to do things like placing embargos on the sale of oil and scrap iron to Japan.  Japan felt, then, that it needed to knock out the US Pacific Fleet so that it would not interfere as Japan tried to conquer an empire in Asia.  This is what caused Japan to attack Pearl Harbor, which means that this is a major "why" of US involvement.

The Japanese sneak attack on Pearl Harbor in Hawaii on December 7, 1941 was the impetus for the United States to declare war on Germany and Japan. The nation had held off joining England and its allies up to this point, but when Japanese aircraft suddenly appeared, attacking a large number of U. S. Naval ships anchored at Pearl Harbor (headquarters of the U. S. Pacific Fleet), President Franklin Delano Roosevelt shortly declared war on the aggressive Axis powers. Nine ships were sunk, many others were damaged, and there were more than 3500 casualties.

How and why did the United States enter World War II?

The United States entered WWII in order to save the world from the totalitarian regimes in Japan, Germany, and Italy. Although the United States was not involved in the war from its beginning in 1939, it was helping arm Britain against Germany through the Lend-Lease program. American destroyers were also being used as escorts in order to ensure that munitions made it to Britain. Before the United States officially joined the Allied cause in December 1941, its destroyers had been fighting an unofficial war with German U-boats for months.  

The United States officially joined the war after the Japanese attack on Pearl Harbor on December 7, 1941. The United States had long condemned Japanese aggression in China and started to embargo war supplies such as aircraft fuel and steel to Japan. Japan saw that it would go to war against the United States at some point in the future, so the Japanese launched a surprise attack against the American Pacific Fleet located at Pearl Harbor. The attack killed over two thousand Americans but failed to destroy vital oil reserves or the navy's aircraft carriers. After the United States declared war on Japan, Germany and Italy declared war on the United States, thus bringing the country fully and officially into the worldwide conflict.