C. It's their problem
The US followed a policy of Isolationism, in which the US dealt with her own problems, while they left the world to theirs. You must note that the US had a mixed population, which means that those with German, Austrio-Hungary, or any other Central Power descent believed that the US should help the Central Powers, while the other half supported the Allied Powers. However, a couple of events led to the US joining the Allied side. These events include the sinking of the Lusitania by a German submarine that costed the lives of 128 Americans, as well as the Zimmerman telegram, which promised Mexico help to retake what they lost in the Mexican-American War. This then led to the inclusion of the US in WW1 on the Allied side. The rise of dictators occurred later in WWII, and again the US took to a policy of Isolationism, however, the US started to supply the Allies (showing a more Allied-leaning side). The bombing of Pearl Harbor by Japan, an axis country led to (a), the US trying to stop Axis expansion.
