HANOI, Vietnam — A year after Vietnam elevated its relations with Washington to the highest diplomatic level, an internal document shows its military was taking steps to prepare for a possible American “war of aggression" and considered the United States a “belligerent” power, according to a report released Tuesday. More than just exposing Hanoi's duality in approach toward the U.S., the document confirms a deep-seated fear of external forces fomenting an uprising against the Communist