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It's cheaper. The U.S. military budget is about $600 billion per year, far more than needed for space settlement.
Fewer casualties. America has lost only 17 astronauts in 45 years of spaceflight, compared to over 4,000 American soldiers and a lot more Iraqis and Afghanis in the current wars alone.
More real estate. The largest asteroid has materials sufficient for 1g living area equal to a couple hundred times the surface area of Earth (distributed into many colonies).
More energy. The readily available, completely reliable solar energy in space is 2.3 billion times that available on Earth. A concerted effort to gather solar energy in Earth orbit and beam it to Earth on microwaves would cost less than the Iraq war and could end oil's energy dominance forever within a few decades.
More money to be made. One small Near-Earth Asteroid has about $20 trillion (yes, that's with a 'tr') worth of precious metals.
And perhaps most important, no one need be killed, maimed, or oppressed and no ecosystem need be destroyed. Nothing is stolen -- everything can be created from rock and radiation at no one else's expense.