The problem will be anticipating what is considered 'war'. Britain has been to 'war' six or so times in the past eight years (I think it is) but they are all off-shores.
For the next couple of decades, we can expect that 'war' will continue to be the local skirmish and local Iraq/Israel sort of thing. Another (truly) global war is probably some way off.
Can GPS be switched off/degraded sat-by-sat or is such a change system (world) wide only?