We could have colonized Mars with the money we spent on the Iraq war -- what else could we do?

Charlie Stross is hosting an open conversation on other things we could have bought with the money wasted in Iraq: for example, we could have sent a colony of over 500 astronauts to Mars, provided modern nuclear power to the USA and shut down its coal plants, built modern cities for 600,000,000 Chinese people to live in, and so on:

For $6Tn we could buy a lot of juice – a quarter of our global civilization's energy budget would go carbon-neutral at a stroke. (Yes, we just solved our carbon dioxide emissions problem by switching to a nuclear economy.) This probably isn't the ideal way of dealing with our environmental problems, and it's a naive treatment of the costs (has anyone done a proper treatment of the economic implications of shifting the planet over to a nuclear economy, say to the same extent as France?) but it's thought-provoking.

Finally, there's all the other little stuff we could solve by pointing $513Bn at it, never mind $6000Bn. Eliminating childhood diseases in South-East Asia? Piffle – Bill and Melinda Gates are trying to do that out of their pocket lint. Build first-world grade housing in shiny new cities for 600 million Chinese peasants, nearly a tenth of the planetary population? Yes, this budget will cover that. What else?

Yes, I'm asking you: what would you do with the cost of the Iraq war (take your pick: $513Bn or $6000Bn) in your budget? Colonise Mars? Solve our carbon emission problem and fix global warming? House half a billion people? Or something else ...?

(And what isn't going to happen now, because we pissed it all away on the desert sands?)