If you want the authorized user to be the one to determine when the gear should (and should not) be sacrificed, it has to be "destroy on command".
Who says the authorized user should? If the equipment needs protection because it can give an enemy an advantage, and it falls into enemy hands, then it should be rendered useless.
That would hand The Enemy an easy method of sabotage. All he would need to do to cripple your gear is to try to use it
Or destroy it with explosives...
It IS in the hands of your enemy; therefore, they could do whatever they like with it, including hiding it somewhere you won't ever find it.
Or if it's based on time elapsed since the authorized user was using it, just keep you away from it for that long.
Your enemy separated you from your equipment... that means the enemy is in control, what more could be said?
There are really two risks, that would cause you to want to block access to equipment:
1. An enemy can pick it up and start using it -- for these situations, a password, or login makes the most sense.
A self-destroy mechanism doesn't really provide meaningful assistance against this threat, because an access restriction is
good enough, as long as it can resist attacks that don't involve days with a logic analyzer.
And for this situation; if the authorized user doesn't think it needs destroyed, then fine.... keep intact, but deny access.
2. An enemy can pick it up and bring it home -- bring it to a lab to analyze; analysis of the electronics might reveal information the enemy
could use. This risk warrants the implementation of destruct mechanisms. Because denying access on the field doesn't prevent analysis.
Moreover, if the destruct isn't timely, the enemy might learn how to disable the destruct mechanism, SO the forces cannot afford to let the
authorized user make a decision (the authorized user may be incapacitated); the destruct should be automatic, and designed to occur in a way
and at a time, which will ensure that the risk of the destruct being disabled, or analysis has already started, will be minimal.
A start, would be to place electronics in rugged tamper-resistant outer and inner cases, which would be opened during normal use without a second thought --
and if the outer case is open, an authentication timer starts counting down....