This can be attributed to the fact that the US went to the war in order to tame the expansion of communism. They did not meet their objective, which means that they lost the war.
North Vietnam may have registered the highest number of casualties, but given the technical knowhow with the Americans, the latter should have easily achieved their target. The Americans are better trained and well organized but they failed to defeat the North in the time that they initially stipulated. The fact that Pentagon resulted to censoring news in order to deceive the public shows that the Americans had accepted failure. The war was started in false pretences and it should not have taken place in the first place.