If you count Japan's actions in China and Manchuria then the war started in the early 'thirties. As for Germany, Hitler declared war on the USA on the 11 Dec 41, along with Italy, as Hitler, although not legally bound to support Japan if they were the aggressor, believed that the USA would "beat him to the punch" and declare war first.
I don't know where the idea that the USA entered the war to stop the USSR invading Western Europe came from. In December 1941 Germany was deep inside Russia, and it took the combined Allied might of Great Britain, the USA and the USSR nearly four years to defeat them.
The USA certainly recognised the threat that the Soviets posed to Western Europe, and the formation of NATO demonstrated both American support for Europe and it's major role in it's defence.