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.
American efforts to prevent Japan from getting an empire that helped cause the Pearl Harbor attack.