The war was lost before it was fought
The film shows how divided the foreign policy team of Johnson's Administration was. People like George Ball was opposed to intervention while Robert McNamara was the driving force behing escalation.There was no clear strategy.Johnson tried to find a middle way without actually declaring war.
The Cold War mentality meant that Vietnam was seen as crucial in the fight against Communism. All politicians , including LBJ, JFK and Richard Nixon bought into the domino theory.
What is amazing is that no one calculated that the North Vietnamese/VC were prepared to fight America despite the technological adavantage that they had.
The Americans made the cardinal mistake of failing to understand their enemy.The American armed forces are not equipped to fight an endless guerilla war in hostile terrain.
Lyndon Johnson, for all his intelligence and political skill, did not what understand what motitivated Ho Chi Minh.The film depicts him being frustrated that Ho won't sit down and talk with him. Ho would not do that on America's terms. so any financial inducement or development aid was not going to work.
With aid from China and Russia,Ho knew that all he had to bleed the US until weariness and frustration took is toll.
Massive bombings, napalm, and search and destroy missions were not going to defeat an enemy that had previously fought the French, the Chinese and Japanese.
Robert McNamara realises that for all his calculations and assessments, the war was a catastrophe. The scene at the end when he resigns shows a man cracking under the strain of a war that he advocated and implemented.