Additionally, by having a strong presence in the middle east, the US create a distraction for their enemies far away from home soil. Also, it would be ridiculous to say "Saddam is dead" and then leave in the middle of civil war, if they wanted to simply kill him, they could have done it covertly without invasion. They have to support the new government and infrastructure that they have invested so much in.
In the grand scheme of things, Mugabe's regime affects only about 12 million people. There are 27 million in Iraq and the theoretical threat of terrorist or sovereign nuclear attack threatens more still.
And with that, let's return to the topic.
This country can not survive. By evicting landowners, they have incapacitated their ability to feed and employ themselves. The supposed intent was to rationalise the power of the white few, but by creating a void where there was wealth, the power and money is left with an even smaller section. While the average life expectancy is below 40, you can expect that these powerful few are maintaining quality of life if only by being beyond the reach of the AIDS epidemic, which although in decline worldwide, is raging on in southern Africa, with almost a quarter of Zimbabweans infected. Of course, after an economic collapse, their money will be worthless. It will be interesting to see how many supporters Mugabe will have when he can't even pay them.
I also recommend watching this presentation on the myths of the third world.
http://www.ted.com/talks/view/id/92