About Vietnam, they didn't undergo a massacre when we left. There was some political instability and violence of course, sure, but what else can you expect for a country that had just gone through a 25-year brutal colonial and civil war? Things don't magically fix themselves overnight.
Cambodia is the place where the massacre happened, and the conditions that sparked that was due to the instability that we caused by the massive bombing. You really have to consider our own bombing as part of that massacre. When we bomb a country, we tend to kill some people, because we are good at it and have really big bombs. Cambodia wasn't better off when we were there, is what I'm saying.
About Iraq. There are two categories of reasons you can use for why we should leave as soon as possible:
1) Leaving Iraq is best for the US.
a) We are going bankrupt by staying.
b) Our soliders lives can be saved by leaving.
c) Al Qaeda is growing in strength due to the US occupation of Iraq. Our own intellegence agencies confirm this. It's because people are pissed off when we bomb them and want to join a group that is committed to fighting us.
d) Our national security is at risk because our military is wearing itself down in Iraq. Deaths of our soliders are up, recruitment is way down. AWOLs and suicides are also way up. Our military gets weaker every year we stay.
Even McCain (if he were to be elected) will be withdrawing at least some of our troops from Iraq. There really is no other choice due to the state of our military and of our budget. He won't withdraw all the troops, of course, but a significant portion of them leaving is inevitable.
2) Leaving Iraq would be best for the Iraqis.
a) They get their country back. Right now, Iraqi is under occupation from a foreign power (us), and the Iraqi government has no power whatsoever. How would we feel if we were under a military occupation and had no control over our own country? I think we'd want our country back.
b) Al Qaeda has no capability to "take over" Iraq. There are a few thousand of them, compared to I don't know how many million Iraqis. There is no possibility of this happening. What are we worried about if we leave? Iran is a much bigger threat than Al Qaeda, but, the Arab League would not permit Iran to attack Iraq after we left. Saudi Arabia is friendly to us and is now armed to the teeth with the weapons we sold them. Any move by Iran would be countered without us having to do anything.
c) Civil War in Iraq is inevitable whether we stay or leave. The best thing for the Iraqi people is just to get it over with as soon as possible. If we stopped arming and assisting the Sunnis, the Shia (who are like 70% of the population), would simply take power. The Civil War would be pretty quick without our interference. Are we worried that the Shia would oppress the Sunnis, an ethnic minority? Sometimes, you reap what you sow. The Sunnis oppressed the Shia for a long time before we kicked Saddam out of power. Besides, there are plenty of places with oppressed minorites in the world, why do we care so much about this particular one? Also, ironically, it's the Sunni who want us out the most, by some polls. so, if they don't want our "protection", why are we staying there??
The opinion polls of Iraq show that a large majority of the population wants us gone somewhere between immediately and a year from now. The polls show that a majority of Americans want that as well. So, who exactly are we staying in Iraq for? The American and Iraqi people? Doesn't seem like it. More likely, we are staying there for the board of trustees of Exxon/Mobile.