Usually at some point in the conversations we get around to this idea. But its success rate is not much better. We left Afghanistan alone the first time and ended up with 2800 dead American's in New York. We could leave Iran alone ... not sure that would work out for us either.
The reality is that we meddle in the affairs of other countries all the time. That is why we have the Instruments of National Power (DIME). At least since 1945 and well into the foreseeable future we try to mold the actions of other countries to meet our desires. Iran wants a Nuke, we work with others to impose sanctions; North Korea tests a Nuke, we stop giving them fuel oil or food; China does something we don't like and we tell them please stop that. Why would it be any different just because we invaded the country or because we are involved in a stabilization or humanitarian operation in that country.
If it is our policy to promote democracy (which it was the last time I looked) then we might want to learn a little more about how and why that happens. Why are other paths chosen; what social, cultural, or economic conditions led to these choices and why did they work or fail?
I am not sure what Cambodia tried I would call modernization but you are right; there were a number of times social engineering was attempted with various levels of effect. I would also agree that a multi-generational model is probably the most realistic. I would argue that what Russia, China, and Japan did were all success of a kind. Russia when from a weak empire to one of the world's superpowers; China is an economic powerhouse; and Japan went from an island nation to controlling most of the Pacific. This kind of thing happens. The question is why did it work for them?
While the generational approach is more realistic (and even that requires managing) politicians are not going to wait generations for changes to occur in countries that make them more amenable to American desires. These political transitions happen. The trick is to figure out why and what the result is likely to look like. Look at the Arab Spring. Everyone is expecting democracies to emerge but there have been plenty of occasion where, once the people have been given the vote they freely vote for autocratic tyrants or theocratic systems. Egypt is already headed that way. I suspect Libya is not to far behind. It happened in Palestine. Democracy will not always yield a liberal political system which is really what the policymakers are talking about when they refer to a democracy.
This is a very complex area. It is one that needs to be discussed, if for no other reason than for those of us in the military to be able to explain to our civilian masters why it is a bad idea to try this type of social engineering again.
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