Had George W. Bush been president in 1938, I don't think it can be assumed that he would have joined forces with Chamberlain, or that the US would have been among the Allies. Of course, that question just begs the conundrum of nature vs. nurture. Is Mr. Bush a product of his intrinsic ideology or his times, and can the former prevail a priori? If so, given his self-assured arrogance, and the kind of men he places in seats of power, WWII may have looked much different.
Of course, I prefer to believe that the surpassing goodness of the American heart would've won out. But isn't the heart of the German people also good? Oh what a world it might've been. And still might be.