A 'good' war
World War II was fought undeniably in self-defense. No westerner or Russian feels guilty over celebrating the victories of 1945. This was not a war of imperialism, but a good war. The Americans who fought are always portrayed as naive farmboys, the Brits as reluctant warriors, and even the stereotypes of "evil Germans" have faded over the decades as societies have come to distinguish between the actions of regular army units and those at extermination camps.
There were no complications in World War II. Poison gas wasn't an issue - Hitler even experienced an attack himself during World War I and knew using it would only result in retaliation and unnecessary suffering by both sides, and of course it was unthinkable for democratically elected leaders to start that. Somehow, even Stalin resisted the urge.
There was barely a camera per army. There was no one to record atrocities. There was no such thing as an atrocity at the time - the enemy was Germany. Not the German army, but the entire nation - that's total war. Allied acts like the fire bombing of Hamburg or Dresden, which in their short tenures exceeded all the casualties London sustained during the Blitz of 1940-41, were simply acts of war. Civilian populations died, but these were civilians in a total war - whether they made razor blades, farmed food or produced munitions, they were helping the German war effort. Civilians, especially in Poland and Russia, were as restless and rebellious as those in Vietnam. The same Russian or Pole a German soldier saw helping an old lady across the street might be the one shooting him at night.
The conflict in Korea still belongs to the World War II-era. There was nobody there to report what was going on. It was a noble war - saving the poor people of Korea from communism - and while concerns about casualties and the possibility of war expansion were present, the war had support. Of course, Korea was a much smaller war, not nearly as epic or against a powerful a foe as Germany - not to mention we could never claim outright victory. It has never held the mystique of World War 2.
Vietnam was too a noble conflict, at least at the outset. A similar situation to Korea, but it had extra complications. The United States, in its efforts to thwart communism, supported regimes of questionable worth. Ho Chi Minh had fought for Vietnamese autonomy and later independence all his life - first by petitioning France, the colonial power in Indochina, then at Versailles in 1919, later by courting the US during World War 2. Eventually, the Vietnamese rebelled and won a war against France. Ho Chi Minh was the popular leader, not those in the South, the Republic of Vietnam. Of course, the political complications of needing France as an ally and keeping communism down had US supporting French regimes and later the South Vietnamese government, neither of which was particularly popular.
Atrocities were committed in Vietnam by both sides - there is no doubt. My Lai is the most obvious case but undoubtedly there were more. In a war where villagers smile at you in the day day and plant traps near your base by night, it becomes difficult to distinguish civilian from enemy. It's an almost unwinnable situation unless you're willing to adopt extreme, genocidal measures. Furthermore, Vietnam was reported on. Daily reports, briefings, updates. That's how TV ratings were achieved - show the most shocking story. There were no more carefully choreographed camera crews filming happy news reels for home. The realities of war were shown. As the nations of Europe learned after World War I - there can be no romantic illusions of war when the full horror of it becomes evident.