Because Johnson was thinking in terms of conventional warfare. He thought that because his army was larger, his air force was tougher and better trained, his navy was massively larger, the equipment provided to his troops infinitely better, there would be no contest.
He did not understand 2 things about Vietnam.
1. He forgot that the Vietnamese had been fighting the French (and beating them). The French in effect had changed the Vietnamese into battle hardened soldiers.
2. The Vietnamese fought a guerrilla style of warfare. The came the delivered hard jabs and disappeared into the night. Conventional war tools don't easily adapt to that kind of warfare.
During the 19th century, the United States ability to remain isolated from European affairs was largely due to its "geographic location," since it is bordered by the two largest oceans in the world.
This of course depends entirely on the person in question, but generally speaking people who believe in such things would want to come back as animals somewhere at the "top" of the food chain.
