Because the United States had learned, at the beginning of the 20th century, that war is incredibly destructive and terrible, and senior, responsible people (like Pres. McKinley) who had seen the South during and after the Civil War (in 1870, Shelby Foote told us on Ken Burns's documentary, 20% of the state budget of Mississippi went for prosthesis) decided that it would be worth a lot to us, never ever again to see a major war fought on our territory.
Capiche?
Now Ron Paul has unlearned not only that lesson -- through his bloodymindedness, which will NOT be convinced or swayed -- but also the lesson of the 1930's: Give people like Hitler and Stalin pieces of Poland, the Ruhr, and Czechoslovakia, and sooner or later they'll come to see you.
I don't agree that you can say what Putin has done means he will take over all of Europe and then try to take us too. listing here
It is a faulty conclusion to reach - because Putin has annexed Crimea, and a few other places before that, that he now will go on to invade all of the former USSR countries, etc etc etc.