I like to date Vietnam War years as After Westmoreland: A.W.

I like to date later Vietnam War years as After Westmoreland: A.W.  (numbers are approximate)

After Westmoreland the US gave up on his truly murderous strategy of killing our young men to kill more of theirs: attrition, against a Ho Chi Minh who would have been willing to see everybody in Vietnam croak as long as the last breath was taken by a communist (not much of an exaggeration).  We suffered stretches of bringing home 3,000 body bags a month -- the goal being, in Westmoreland's own words, to reach the "cross-over" point at which the North could no longer replace soldiers faster than we could kill them (his own words!).

That's the war everybody thinks we waged -- and lost!

3 1/2 A.W.  The US ambassador to Vietnam could now drive anywhere in the countryside without a military escort: the payoff of counter-insurgency.  The Viet Cong guerrilla army had been reduced to totally non-Southerners, which in turn left NVA main force units (those guys you see in We Were Soldier Once and Young) eating grass and without ammo.

North Vietnamese Army main force units -- hiding most of the time like guerrillas -- required supplies of food and ammunition to be propositioned in hidden caches before they could mount an attack.  After the countryside was wrested from VC control it was no longer practicable to cache supplies or maintain cave systems.

It was easily practicable to "win the hearts and minds of the people" simply because the Viet Cong were so hated.  The movie Full Metal Jacket portrays the Viet Cong massacre during the battle of Hue: calling in police and teachers and government office workers, etc., for "re-education" and then shooting them in mass graves (bodies depicted under white powder).  The movie said the body count was twenty,  Wikipedia calls the civilian and POW count possibly as high as 6000!

Beginning with his struggle to take over the North (more on that below) Ho and friends only approach to recruitment was to kill and kill and kill anybody who didn't see things their way.

A.W. 4 1/2  The North tried an out and out Korea style conventional invasion -- beaten off with 50% casualties.

A.W. 5  The US Congress cuts off the money and supplies the South needed to continue the war along with US air support -- all reduced to a trickle.  The South begins rationing bullets and artillery rounds.

A.W. 7 1/2  The North -- having harvested a few more crops of eighteen years olds -- finally sent them to overrun the US Congress-disarmed South in six weeks.

Two million South Vietnamese anti-communists fled the advancing forces -- one million by boat.  Good thinking; the North rounded up and shot 130,000 who did not.
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In the 1950s, Ho Chi Minh's began his life long approach to winning supporters typically by sending terror squads into villages, lining up some boys who may have joined some government program (e.g., education) and shooting them.  When that village was rendered sufficiently docile a maintenance squad would come in and the terror squad would move on the next village (ink blot tactic).

(N.B.  Vietnam was no longer a colony in 1949.)

After Ho won the election in the North -- population 13 million; 98% of the peasants owned the land they tilled (70% in the South) -- Ho shot 50,000 peasants as capitalist exploiters and sent 100,000 to re-education camps.  Had some formula according to how many pigs farmers owned, etc.  (Communists have to have their land reform -- even they admitted they may have gone too far.)  A year later Ho's own home province rebelled -- the army crushed this, killing 6,000 peasants (wounding?).  For the early Ho, see: The Two Vietnams by Bernard Fall -- a French political scientist who was allowed to move freely about the North while the fighting was still going on there.  Killed when his jeep hit a mine in 1967.  I found his book too dry and scholarly to read when I was in my 20s.

[Late addition: a novel by a North Vietnamese veteran whose Glorious 27th Youth Brigade went out with 500 and came back with 10 alive: The Sorrow of War by Bao Ninh -- at first banned in the North but now reportedly allowed.]
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Why were we in Vietnam?  Why did we let millions of Koreans die defending their rice and fish exporting peasant culture from collectivization -- when Russia was still on it's back, it's dictator not moving aggressively, only wanting to be Czar; and Mao not really known yet?  Why did we bomb populated cities (!) in helpless Japan?  

We bombed Japan out of force of habit.  If we had never bombed Germany -- in desperation to keep it from taking over the world -- it never would have occurred to us to bomb a single populated city (!) in Japan.

We defended Korea much out of the same force of habit.  Having just fought off two little countries (with single time zones and insufficient natural resources which ironically motivated their aggressions) from taking over the world, we just naturally took on world communism -- even though it posed no critical threat yet and even though the end of the Korean peninsula offered no place else to go.  That happened to turn out just fine.

By 1965, Vietnam's decision year, communism was at high tide: the two biggest countries in the world were coming at us ("We will bury you", "Your children will live under communism" -- for those who weren't around for Khrushchev) with bombs that could literally leave a half-mile deep crater where Hiroshima used to be.  Very scary time.  Khrushchev was a true believer who challenged us everywhere (Berlin grab, Cuban missiles); Mao was as bloodthirsty as Ho (see above) -- and we feared that if the democracies just let Ho run over South Vietnam it would open up the whole world to communist aggressions

Realistic?  ???  At least there was a plausible rationale.  Also Lyndon Johnson couldn't back out of Vietnam and keep his voluminous domestic legislation moving safely past "good old boy" Southern US Senators.  What a choice.

By 1975 we had won globally -- communism was receding worldwide -- we could afford to lose locally.  We, not the South Vietnamese.