<<Then educate me. Why was the causus belli just for WWII and not for Viet Nam? >>
WWII was a war that the U.S. entered late between Nazi Germany and its allies on the one hand and France, Great Britain, the USSR and their allies on the other. The causus belli was German aggression which would have led to the domination of Eurasia by the Nazis and their takeover of the British and French Empires. This would have inevitably led to the domination of most of the world by the Nazis which most people generally would tend to view as something very, very bad. If you don't understand that, I would suggest some basic courses on European history in the 20th Century, but it's a huge topic which is not of much interest today simply because it is so widely understood. Most Americans would have considered Nazi domination of the planet a bad thing.
Viet Nam was primarily a struggle for national independence from the French Empire, which FDR had supported in principle, having told the Allied European colonial powers (Britain, France, Holland, Belgium) that their colonialism was contrary to the principles of national liberation for which America was fighting in WWII and that consequently the U.S. would not support their postwar efforts to retain their colonies by force. Although France continued to fight the NLF (the communist-led National Liberation Front) after the war, a symbolically important battle loss at Dienbienphu convinced the French to pull out in stages, leaving the North in the hands of the NLF and the South in the hands of a puppet government they had hand-picked prior to their departure from their former colonial collaborators, most of whom were Roman Catholics in a predominantly Buddhist country. In short, what happened was that the French puppet government, having practically zero popular support, with the help of influential RCC and anti-communist pressure groups in the USA, managed to sucker important American politicians into supporting their so-called anti-communist struggle as part of the Cold War. Elections were to be held two or three years after partition, but the South, with U.S. support, cancelled the elections, which the NLF would have won in a landslide, due to the popularity of Uncle Ho (Ho Chi Minh.)
Thus the real objective of US interference in Viet Nam was to prevent the free elections promised to its people, which in itself was distasteful to many Americans. The official causus belli was the fake attack on American warships in the Tonkin Gulf, which many Americans knew or suspected from the beginning was faked, much the same way as the "threat" of Saddam's "WMD" was faked years later.
Both Viet Nam and Iraq were sparked by manufactured causes, and in both cases the American people were lied into war by deceptive and manipulative politicians. Although many Americans are ignorant, belligerent, racist ass-holes ready to kill and torture brown-skinned or dark-skinned people for no good reason and believe everything their government tells them because of their sheer stupidity, many Americans are good, smart, decent people who can sense when their government is lying to them, have no real desire to kill people just because of their race or religion and will rightfully resist any call to sacrifice in a cause which, whatever their government tells them, they realize is a crock of shit. Even the moron racist ass-holes at some point when the government's lies become more and more apparent, finally start to realize that they have been had, and begin to waver in their support of a campaign which they had been led to think would be a cakewalk. Since it is from these moronic ass-holes that most of the government's troops will come when the smarter half realizes that the cause is no good, these guys are the first to see the real results of the government policies coming home in coffins and wheelchairs, and this could be considered a part of the Great American Awakening, when the entire nation slowly comes to realize that, once again, their leaders have turned out to be a bunch of lying bags of shit with nothing left to offer and no more promises that anyone is ready to believe in, not even the morons. This is when the plutocracy that really rules the country comes to the conclusion that the plug has to be pulled come hell or high water. The game is up.
There was the america first crowd during WWII, part of the reason we entered late, why were they wrong?
