Well, it appears you're not an apt student of history. Things didn't take place as succinctly as you've stated, and not only that, the events that led up to, contributed to and brought about the Second World War had been brewing for decades prior to it actually happening. WWII is in many ways a continuation of WWI, but its origins can be traced back even into the 1800s. Also, the world today is a vastly different, more complex place than the complexities of that earlier day.
It is a fallacy to simply assume that when the United States entered into World War Two that it sealed the fate for the Axis powers. That may have been the final outcome, but the US neither acted alone nor was successful in every endeavor that it undertook. The US government was heavily involved both directly (openly) and indirectly (clandestine) in world politics prior to December 7th, 1941. It's true that the Axis did not want to grapple with the US in addition to the other countries it was already fighting, but German, Japanese and Italian governments also believed that if the US did enter, it could be beaten just as easily as those other countries. Remember, at the beginning of the war, the Axis powers were largely victorious in almost every country they invaded and in almost every military action they initiated. Their sense of self-confidence was high, but they were also wary of the possibilities of what would happen to their overwhelming advances if the Allies received more and more help. Both the Axis and Allied powers each had dozens, hundreds or thousands of people carrying out espionage inside of enemy territory. It was a cat-and-mouse, spy-vs-spy game that has always existed then and still continues today. The help of the US was needed and useful, but the US did not act alone in defeating the Axis.
Fast-forward to all world events that have taken place since April and August 1945 (the former is when Germany surrendered, the latter is when Japan surrendered). The US has friends, enemies, and neutral countries, and those three lists change all the time. The US has been involved militarily and/or had a large military presence in Japan, Germany, the Philippines, Australia, Great Britain, France, Poland, Korea (now two separate countries, North and South Korea), Cuba, Vietnam (then two separate countries, North and South Vietnam), Cambodia, Laos, Thailand, Panama, Grenada, Kuwait, Iraq, Afghanistan, etc., etc., etc, just to name a few major ones. Not all of that contact has been positive nor well-received, and not all of it went the way the US government wanted it to (to state it lightly). Not everyone in the world agrees with the US government's policies and/or influence in world affairs. Some countries, and even some non-governmental entities, openly and continually challenge the US. Some of those challenges are less than peaceful, and some of the US responses have been less than peaceful.
The US is not always the good guy. The US may contribute to some peace in some places sometimes, but it is also responsible for a lot of war in a lot of places a lot of times. If you're going to write about world events, it really helps to do so from a truthful perspective.
