No, the answer is because their (the Fascists that is) previous actions had given America cause to be a threat.
Poster ljadw has presented the arguments regarding the German invasion of the Soviet Union that...
(a) the invasion of the Soviet Union was the right strategy,
(b) because it was the only way to prevent a war with America, and
(c) the impetus for this epiphany moment is linked to a GOP convention in August 1940 (that doesn't seem to have occured!!!).
If, as poster ljadw alludes to, America was at the very top of Germany's worry list, so great a worry that they were prepared to gamble all on an attack on the Soviet Union, perhaps the "ideal strategy" would have been to refrain from trying to conquor Europe in the first place. The annexation of Austria followed by the takeover of Czechoslovakia did not go unnoticed. The invasion of Poland brings Britain and France into the war. The former being a key link in poster ljadw's theory.
The "ideal strategy" for the world was for the Fascists, in particular the Nazis in Germany, never to have come to power.
The "ideal strategy" for the Facists to avoid destroying their own countries was to not have a policy of trying to conquor other states and people in the first place.
I see nothing "ideal" in a world with Hitler et al masters of Europe and the world. Do you?