It's amazing how easily low tech can beat high tech. The US was able to dominate the battlefield militarily but wasn't able to maintain security. The old "Army Doctrine" of superior, massed firepower worked wonders in conventional warfare but not against a small, determined force. The Vietnamese were no strangers to combat. The fought a very successful campaign against the French colonial forces. Small unit tactics like tunnel rats and continuity operations (?) like what the Marines did during the harvest paid off big but didn't fit into the dominant paradigm and never received much support. As far as the tunnel rats go, didn't the Australians lead the way in that field?