Why did US enter the second world war?

When Japan extended its support to Hitler and bombed the US base at Pearl Harbor, the US entered the Second World War.

Part of the reason for going to war against the Germany, Japan and Italy. In March 1940: the Lend-Lease act was passed stating Roosevelt could direct aid to whomever he wanted. This meant that the US was no longer neutral. The US ended up contributing 42 billion dollars to the war effort by 1945. With Hitler taking over Europe, FDR saw fit to aid the British and French troops with arms, etc.

It all started many years before Pearl Harbor was bombed. It starts with the Japanese invasion of Manchuria, China. The Japanese invade Manchuria on September 18th, 1931. China was in the middle of a depression (much like our own) so the Japanese quickly get a strong footing in Southern Manchuria. At this time, the United States is good friends with the Russians. It starts pissing off Russia that the Japanese are taking over China. If it pisses off the Russians, it pisses off us. Also, the Japanese invading China violated the Kellogg-Briand Pact of 1928 which renounced war as national policy. So, the United states tells Japan if they don't get out of China, we'll stop trading oil with them. Japan stays in China, so, naturally, we stop trading oil. Eventually, Japan is forced out of China, but not without a lasting grudge against us Americans. Of course they hate us after this, so when Pearl Harbor was left for an entire day with everyone on holiday, they bombed the Hell out of it. America was looking for a very good reason that everyone could agree on to join the war. FDR had been wanting to join the war all along, and when the Japanese bombed us 10 years later, it had everyone in such an uproar that no one would back down from the fight.
This is one of those questions that has exercised people for a long time, and as with many important events, has attracted some strange theories. The basic facts given above are correct. The fundamental cause was indeed the bombing of Pearl Harbor. Such an attack is tantamount to a declaration of war. It's also true about Japan's invasion of China, and America's oil embargo. America had good relations with China (which was not communist) at that time. That was probably more significant that US-Soviet relations. The theory that the President knew about Pearl Harbor in advance has been suggested many times. There is some interesting evidence that there was some advance knowledge (or possibly that there ought to have been) but pretty much all reputable historians agree that the President did not have definite information about the attack. It's not true that Germany declared war on the US before Pearl Harbor. They did a few days afterward. It is just possible that, had they not done so, the US would have refused to declare war on Germany but fought only Japan. It also should be understood that while the US was technically neutral up to Pearl Harbor, they were already providing substantial aid to the Allied side. The British were being provided with arms and supplies on favorable terms, and no such supplies were being sent to the Axis. Convoys across the Atlantic were being escorted by US warships for the Western part of the journey.
The US was already trading with other countries through the Lend-Lease Act, a document allowing the US to "lend" countries like Britain war materials in exchange for money. Surely the Nazis knew this, but the real thing that got us into the war was Pearl Harbor. Of course there are theories that FDR knew about the attack on Pearl Harbor and didn't do anything to defend against it for the sole purpose of justifying entry into the war, but I don't believe that. Japan saw our navy as a threat to it's imperialistic desire to have power over all of Asia and the Pacific. The same US navy, which was conveniently, yet threateningly positioned nearly half way between the US and Japan. Japan also didn't have very good control over its military leaders, as General Tojo took his own action by attacking at Pearl Harbor.
It is a popular misconception among Americans that the US voluntarily entered WW2, at least against the Germans. In fact, the US didn't. The US entered the general war as a result of the attack on Pearl Harbor. But the US entered against Japan and did not, repeat not, declare war on Germany. However, a few days after Pearl Harbor, Hitler declared war on the US, thereby putting an end to the US dilemma. After a meeting between Churchill and Roosevelt, it was agreed that the British and Americans would have a "Germany first" policy. Whether the Americans would have declared war on Germany had not Hitler made the decision for them is one of the great unanswered questions of history. The US had, of course, been completely isolationist prior to 7 December 1941 - Roosevelt's Lend-Lease program had got through Congress by one, repeat one, vote. The answer is that yes, probably the US would have entered the war against Germany, but possibly not on the scale that it did, and almost certainly not with Germany being given priority over Japan. But nobody will ever know for sure.
Obviously the attack on Pearl Harbor by Japan triggered a declaration of war and the 'official' entry of the USA into the war December 1941. However, the USA was already unofficially at war with Germany since USA naval forces were shooting to kill German naval units in the Atlantic. The USA was sending 'volunteers' to fight in China against Japan. The USA was keeping Britain in the war by providing 'loans'. Some would say that the USA cutting off 100% of credit to Japan (along with the Netherlands and England) which meant cutting off Japanese oil supplies, was tantamount to a declaration of war by the USA on Japan, Summer 1941. Clearly the USA was gearing up for war even as the population clamored to stay neutral. One way or the other Roosevelt was going to get the USA into the war. The Japanese just made his goal easier to achieve.
The December 7, 1941, Japanese raid on Pearl Harbor was one of the great defining moments in history. A single carefully-planned and well-executed stroke removed the United States Navy's battleship force as a possible threat to the Japanese Empire's southward expansion. America, unprepared and now considerably weakened, was abruptly brought into the Second World War as a full combatant.
Although the US was leaning towards involvement in WWII, many people still saw it as a "European" conflict. That changed on Sunday, Dec. 7 1941 with the Japanese attacked Pearl Harbor in Hawaii. A good portion of the US Navy was destroyed. The sunken ship Arizona still remains there today. The emotional reaction to Pearl Harbor was very similar to the feelings most Americans had after the September 11 attacks on the World Trade Center: Shock and disbelief followed by anger.
U.S. interests had been attacked. U.S. ships were attacked in the Atlantic when Germans didn't like U.S. supplies being sold to England in support of their efforts to defend Europe while Germany was making its plans to gain control of all of Europe as a first step in control of the world. BTW, the League of Nations had placed sanctions against Germany that was intended to prevent buildup of a German military after the end of WW1. That's why the U.S. was involved in the Atlantic. U.S. interests were attacked in the Pacific when Japan was trying to gain control of all islands in the Pacific.
The US provided millions of dollars in weapons, armament as well as money itself to the Allied forces. But the attack on Pearl Harbor pushed us officially into the war.
The real reason we entered WW2 was because Japan did not like the fact that we stopped selling oil and scrap metal to them which they needed. So on December 7, 1941 that attacked us. The next day we declared war on them and in return Germany and Italy declared war on us.
Because U.S. interests were attacked and U.S. citizens were killed.
The United States officially entered World War 2 on December 8, 1941 by declaring war on Japan following the Japanese attack on Pearl Harbor on December 7th. FDR said it would be a day to live in infamy.
The United States got involved in WW2 because the United States got attacked in Hawaii. This is known as Pearl Harbor. The Japanese bombed us so the US retaliated.