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.