Germany was winnable because after the fighting U.S. and other allies we're willing to occupy the area with huge number of troops for 50 years. Also Marshall plan.
Vietnam was not winnable because general population didn't support such high numbers for occupation. There we're practically no U.S. troops left for the last two years of the war.
Since Vietnam U.S. cannot use draft. So U.S. can only conquer one, maybe two countries while keeping Korea and Europe under control. But that leaves those troops bound on foreign soil. Which in turn limits U.S. power projection and therefore cannot be the long term solution.
Wars have not changed. DoD just would love to find a way to win wars without the ensuing occupation. And that's unlikely to happen.
No, as old Macchiavelli put it: if a country is hard to conquer, it will be easy to hold; if a country is easy to conquer, it is hard to hold. The reason is that a united, highly organized country fights hard, but it won't fall apart into dozens of factions after being conquered. Unlike countries like Iraq or Afghanistan.
I'm holding a copy of Prince and happened to find that verse. It's about Ottoman imperium and feudal France.
If there is a dictator and you simply become the new dictator, then keeping a country is easy. That would have been because none of the military leaders would have any loyalty among the people. But nationalism has happened since and it's questionable if this would hold anymore.
If there is large number of Feudal lords, this makes taking France potentially easy as some of them would betray the king. But on the other hand they would have loyalty of the people with them.
Times have changed, otherwise Iraq should be peace of cake now that Saddam is dead.
After Gulf War I, Saddam had to resort to brutal military actions to keep the various tribes from seceding. Afghanistan has been ruled by 'feudal lords' for a long time. Neither Iraqi nor Afghan soldiers were eager to die for their states, which were colonial constructs anyway. If the country you are invading implodes after a few days of fighting, you can expect trouble.
Occupation isn't the key, it is the willingness to inflict civilian casualties to demoralize the enemy to the point they can no longer fight.
We go out of our way to not harm civilians, the very same who likely are supporting the people we are trying to kill and this has the effect of extending the length of any war. There are some who claim inflicting civilian casualties only encourages the enemy but history has proven time and time again that simply is not true.
The Spanish-American war of 1898. The Philippine guerrillas on Mindanao were crushed so savagely and effectively the locals still associate the US with actions taken during that time period.
War is ultimately a contest of wills. It's not that you can't beat irregulars, it's that you have to be so brutal the folks at home won't tolerate it unless the threat is existential.
Still U.S. saw it best to install puppet and continue occupation for about a generation. There we're 20 000 troops in Philippines in 1941. In a country of about 16 million people.
There are over 30 million people in Afganistan and about 13000 Nato troops. If that works out, U.S. has gotten better occupying countries.
Korean war was winnable for the same reason. Huge numbers there. Even today. Here you can see the long tail of Korean war. http://www.heritage.org/~/media/images/reports/chart5_1.ashx...
Vietnam was not winnable because general population didn't support such high numbers for occupation. There we're practically no U.S. troops left for the last two years of the war.
Since Vietnam U.S. cannot use draft. So U.S. can only conquer one, maybe two countries while keeping Korea and Europe under control. But that leaves those troops bound on foreign soil. Which in turn limits U.S. power projection and therefore cannot be the long term solution.
Wars have not changed. DoD just would love to find a way to win wars without the ensuing occupation. And that's unlikely to happen.