USA wars

   As i rad so far, I learned that most of the countries who had war with USA, were in friendship  with it. i want it to know How these friendship turned into wars. is it all the same problem ? what i knew so far is, most of it started with defect of political relations between small countries. But in some way or how, the US government interfere between these countries. For example, the WWII with japan, Vietnam, and Afghanistan. And then everything get missed up and turned against US. so far, i didn't know what is the main reason for all that .