As far as I am concerned, the only new point was brought up by BoomerD regarding Vietnam. As BoomerD noted, Vietnam was played by an artificial set of rules. Even if that set of artificial rules really dated back to the Korean war of the early 1950's.
But to explain what BoomerD was saying, Russia and China agreed, the USA and its allies could bomb North Vietnam from the air, but as soon as USA tried to take an army that put boots on the grounds on North Korean soil, Red China and its massive ground army would intervene on the side of North Vietnam to stop the US ground army. Exactly what had already happened in the Korean war two decades before. And as we all know, that ended in a bloody stalemate.
Of course there was US talk about using its nukes, and when MacArthur advocated it during Korea, Truman fired his ass for even thinking it. Because it would lead to a WW3 nuclear war, even though the USA had the far superior nuclear arsenal and the planes to deliver them that the Chinese and Russians lacked during the Korean war. By the time Vietnam rolled around, WW3 was unthinkable, because all sides had achieved the massive nuclear overkill of mutual assured destruction. With the ICBM's to deliver them.
But still, a war like Vietnam was rather advantageous for all major powers. As a place to test the latest military technologies with minimum casualties to the major powers. Of course for the hapless Vietnamese, be they North or South, it really sucked, as they lost over 2 million people. Which translates to about 7% of Vietnamese population. As the principles of a proxy war became firmly established. And later the USA used the same principles when the Russians tried to take over Afghanistan, as Russia experienced its own Vietnam type lesson.
But still, in my opinion, the BoomerD lesson fails to apply to our two big subsequent military occupation failures. Because in Iraq and Afghanistan, there was no big major power intervention, or setting up of artificial rules. As the US military could go anywhere it pleased in the Iraq and Afghan theaters of war. Nor are the insurgents being armed in a big way by foreign major power. Nor are they test beds for competing major power military technologies. But still the USA is losing in both places. But certainly worse in Afghanistan. Without the foreign intervention of the BoomerD Vietnamese lesson. As the major force we were fighting in all three places was really nationalism.
Or we can look at war as being as old as man kind. As groups of humans and even animals, inevitably fight for territory. But still, war used to be fought by a basic simple set of rules. (1) Offensive wars are especially expensive, one has to arm and feed a large group of men for a long time (2) Have army must travel, with the object being, to fight their way to the enemy capital, and with that mission accomplished, the war is supposed to be over. And then, the real motivation for soldiers joining in wars, they get to loot whatever portable wealth they can find. As the victorious country tries to set up a friendly government. Which usually proves temporary due to nationalism. (3) The defending country usually has the advantages, a shorter supply line, having a large number of natural obstacles like rivers and mountains is an advantage, having a large number of intervening hard to take cities is an advantage, and woe be to a country without friends and allies who will join in mutual defense. And adding Navies just added a sea power dimension to the overall military questions.
But as usual, as technology marched on, the ability to wage war became more deadly and dangerous. As firearms and Canons could level formerly impregnable fortifications,
and as trains, motorized tanks, and personnel carriers were added, armies could move with blitzkrieg speeds. Add in planes and bombs, brings up to the technology used in WW2. The last major conventional war mankind can ever fight without wiping ourselves out as a species. Or we could go back from WW2 and visit the its predecessor, WW1, the so called war to end all wars. As all the shortsighted victors of WW1 did was to plant the inevitable seeds of a future WW2. And what a horrendous convention WW2 was, as the slog to Japan and Berlin, took year after bloody year. As for the first time in human history, winning a war required decimating the civilian population and the economies of competing nations.
But still, as a young boy, born after WW2, why should I complain about war, as the USA emerged with no real domestic infrastructure damage and the strongest economy in the world. In contrast, the Russian really suffered, but still emerged as a WW2 winner, due to the massive land gains they made in central Europe. The big losers were the Western European powers that dominated the world prior to WW1. As they lost all their overseas holding that were the foundations of their national wealth.
As we live in a post colonial world. And now I watch my country, the USA, do the same damn thing the Western Europeans did. As we fight endless stupid wars, and piss our formerly dominant position away. As we fight against the inevitable forces of change, when smarter nations take advantage of changes instead.