It would be something more Pearl Harborish, like a nuke or EMP detonated in space over the US that fries our communication networks and power grid.
In which case, this proposed kill switch would be redundant.
A high altitude nuclear detonation above the continental US is the first strike protocol for any nation-state that would try to attack us. Even then, a lot of our military's core command and control infrastructure has been hardened to withstand such an EMP for decades--as it was known that a high altitude detonation over US soil was the Soviet's plan for a first strike...That is why Cuba was such a big deal. Still, the EMP would completely decimate every non-hardened electrical system.