[Poohbah]

Every nuclear warhead in the US arsenal is fitted with a limited-try Permissive Action Link device designed to prevent unauthorized or inadvertent use of the weapon. After three tries with the wrong code (out of 1 trillion combinations), the warhead will disable itself.

The PAL is the product of the nation's best and brightest engineers, and has been rigorously tested.

Unfortunately, the PAL can be disabled with a penknife, some Crazy Glue, and a stun gun.

[Chancellor Palpatine]

Harriers can sit in bunkers at Ft. Hood, unmaintained for a thousand years. All you have to do is gas them up with fuel that hasn't changed either, give some people who have regressed to a barbarian state a few hours in a simulator, and they'll be expert pilots, able to shoot down alien aircraft of the superior technology that defeated mankind in the first place.