There are a number of questions here, many of which are not directly related to each other. This response will focus on the first question. World War II broke out in the fall of 1939, when Germany invaded Poland. At this point, the United States under Franklin Roosevelt pursued a policy of neutrality, established through a series of laws known as the Neutrality Acts. Though most Americans were generally sympathetic to the British, a strong strain of isolationism existed in American politics—a feeling best exemplified by the "America First" Committee that opposed intervention in almost any case.
As Germany overran most of the European continent, and especially as they began the aerial bombardment of Great Britain that was intended to precede an invasion, the United States crept closer to war, beginning with a so-called "cash-and-carry" program intended to provide war materiel to the British. Eventually this policy, which was actually controversial, was expanded to a "Lend-Lease" plan that loaned military equipment and money to the Allied powers. Still, though, the US remained neutral, though war planners began in 1940 to plan a European war against Nazi Germany. Over time, American public opinion began to consider war as a possibility, though isolationists remained and essentially accused FDR of pushing the country toward war.
The catalyst for American involvement in the war was the expansion of Japan into Asia and the Pacific. Long at war after invading that country, the Japanese Empire occupied French Indochina (modern-day Vietnam) in July of 1941. President Roosevelt responded by freezing Japanese assets and halting the sale of oil to Japan. At this point, the Japanese High Command decided to attack the US Navy at Pearl Harbor. This massive sneak attack on the US Navy base took place in December of 1941 and ended US neutrality (both diplomatically and in terms of domestic politics) by dragging the nation into war with Japan, Germany, and Italy.