Space, to use a clich й , is big. The closest of the Settled Worlds is fifty light years out from Earth. The most far-flung four times that distance, and some of the Colony transports are still going. If some maniac starts rattling tactical nukes, or some other biosphere-threatening toys, what are you going to do? You can transmit the information, via hyperspatial needlecast, so close to instantaneously that scientists are still arguing about the terminology but that, to quote Quellcrist Falconer, deploys no bloody divisions. Even if you launched a troop carrier the moment the shit hit the fan, the marines would be arriving just in time to quiz the grandchildren of whoever won.