The illusion means the wrong perception of something. In the paragraph, the purpose of the illusion word is to emphasize that the United States was never isolationist and powerful completely.
The literal meaning of the word illusion is to think falsely about the things they see, or it can be wrong beliefs. It can be the deceptive appearance of thoughts or impressions.
In the paragraph, the word illusion is repeated to let people know the position of the United States after World War I. It was an illusion of people that the United States had ample weapons and was completely prepared for major military actions.
Over the period of time, the military power of the United States has grown intensely which forced people to think that the US was the only powerful country after World War I.
Therefore, the word illusion was repeated to tell that the United States was never completely powerful and isolationist.
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