and you have to remember Amercia turned up late for both wars and would probably still be sitting on the sidelines if Pearl Harbour hadn't been attacked.
Had Germany won, the British Empire would probably been dealt a blow, furthering the cause of self-determination the United States espoused. The Soviet Union, without much of their population and prime real estate on account of the treaty of Brest-Livotsk, would have been more probable to collapse. And in any case, World War 1 was a European squabble rather than an international affair.
As for World War 2, the United States aided the Allies long before Pearl Harbor (Americans preserve British spellings when part of proper nouns; Britons should do the same). Furthermore, the United States was not as responsible for war breaking out. France in particular was too harsh to Germany after World War 1. And then there was Neville Chamberlain.
The UK declared war solely because they were bound by treaty to do just that. Furthermore, they were in the path of the blitzkrieg. The United States had an ocean separating them from the Third Reich. Germany was not as much a threat for Americans as it was for Britons.