[Tom Bombadil]

How are WE protected against such devices?

We probably aren't, with nothing more than the RF shielding traditionally used in military electronics.

Since our forces rely heavily on "smart" stuff it could be a problem.

[Darksheare]

It gets flown in on a tomahawk missile prior to our movements. Once it's fried all the electronic gear the enemy has, we come in and rape their army by crushing them without humor, pity or remorse.
Sort of a "What do you mean your radar doesn't work anymore? Oh, that's too bad. *BANG!*"