Not to denigrate the war-time exploits of our allies, but where would you be today if the cavalry didn't show up at the last minute? Besides, isolationist sentiment was so strong in this country (and it appears to be growing again today) that it guaranteed that we wouldn't show up until the last minute. There were millions of Americans who exerted enormous influence on Roosevelt's government to keep it from getting involved in what they viewed as Europe's "sordid little war."
It appears that we do not learn from the mistakes of history. Isolationism and appeasement still have their adherents today, who have failed to grasp that these political philosophies cannot guarantee a nation's safety from external threats.
Playing kissy-face with terrorists will not pay any dividends.