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I wonder whether a more effective alternative would be to station troops in space. The Outer Space Treaty prohibits the deployment of weapons of mass destruction in space, but not conventional weapons or combatants. An invasion from space could be made with virtually no warning. I'm not sure stealth would be possible due to the plasma created on entry, but decoys / countermeasures are possible.

With the potential for spacelaunch to become drastically less expensive due to reusable launch vehicles, a constellation of manned hypersonic glide vehicles may become economically practical.