An emp would knock out electrical grids.... sure...
But that isn't annihilation...
Say russia installed this in major cities (rural is acceptable loss), then manufactured super sonic nukes that could deliver a strike in under 5 mins... or even a minute... they could strike many of our nuke sites (not all because of subs) but our major military operation bases, then we strike back from subs, but these take them all out, there's some fallout from atmospheric explosions which may be as bad, but maybe they hoist huge fans to blow it south to the Ukraine or another enemy state...
I don't think EMP's really would deter this sort of outcome... MAD is becoming less and less a thing, super-sonic weapons are the real clencher, because with those it's simply whoever strikes first - if they strike hard enough can disable just about any enemy... so the trick will be not getting to the point of first strike... which...I don't think we can really prevent, not w/ water wars/climate resources diminishing, etc....
The worst war ever will come, we thought globalization could keep people sane but threats to supply chains, and resources, and the existence of the earth itself turn people into desperate people. Desperate people don't always do what's considered 'sane'.
What we'd deem an evil dictator might try and nuke 80% of the population to save the climate, and it probably would succeed but most sane people would say the cost is too great for that ...and they'd be right too.... I mean Thanos had good intentions but horrible means towards them...
A madman who gets elected or inherits the button to the nations military arsenal could decide to burn it all down like Nero...