It's definately a possibility. It's also a possibility that a huge meteorite could strike the planet sometime in the next century and destroy all life.....or that some new disease/plague could spread killing all but a few survivors..........or that I could die in one of the many plane trips I take for work.
If you live your life worrying about all the "what if's" you won't really get to live much will you? I think it's best to plan for the worst (ie: military preparation and homeland security measures) - but keep in mind that anything is possible and right now the world is beautiful - so enjoy it.