Question: Why did American soldiers begin to question U.S. involvement in the war?
Answer: American soldiers thought many South Vietnamese were indifferent toward their own nation. ~Gradpoint
Another thing: Although the United States was superior in military technology and weapons. It lost in the strategic war against the forces of Northern Vietnam. American Soldiers became aware of this throughout the years, as no actual progress was made but the number of American casualties was growing. This led to an effect called "The Vietnam Syndrome", which was an overall reluctance towards the American Army to make further efforts to continue in the war.
Hope this helps!
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~A.W~ZoomZoom44