We have all known a bully. A bully is someone who needs to keep proving he is tougher than everybody else. A bully has a very large but fragile ego which needs constant massaging and the best way to do that is with a regular "show of strength." So a bully must spend his time stalking around looking for opportunities to demonstrate his power over others, to reassert his position of dominance and control, to hide his insecurity with tough talk and bold claims. A bully is especially dangerous when he feels threatened or challenged, when things just aren't going well, or especially if he has just lost a fight. He then becomes desperate to reestablish his position of power and authority, to get back on top, to show who is still the boss, to prove that he is still the toughest kid on the block who ought to be feared by every body else.
The American defeats in Indochina, at the hands of the Vietnamese and the Cambodians, proved that the U.S. was no longer the toughest kid on the block who could always make things go his way. In fact, it was some of the smallest kids on the block that wouldn't give in to the bully's beatings and eventually sent him away with a bloody nose. All this made the bully feel very insecure about his power and control over the rest of the block. He needed a chance to show how really tough and powerful he still was, a chance to show that he was still number one on the block, a chance to again generate the fear and demand the respect he felt were his due.