Well that could be said about any modern country, the only places that wouldn't work is on a 3rd world nation, but who wants to invade them.
But a large part of our military command is shielded against EMP, so they'd still have to deal with them.
Trax, most of our military is hardly close to being hardened for an EMP attack. Command centers might be, but the sheer disaster that a coordinated EMP attack on this country could wreak is beyond any comprehension. At the very least if three weapons were detonated in the atmosphere across the continent, 95% of the civilians would be without any form of modern power or conveyance. Very shortly there after, a massive die off would occur. Fighting back would be pointless especially considering we might not know who perpetrated it, and how then does a nation without any means of conveyance, supply it's military, let alone it's populace used to daily supply.
I was turned onto a book called "One Second After" by some others. It is a highly plausible scenario of just such an attack. A good read if you have the time.