that is why I said “cut all of these cables”
I understand that if only one cable was cut there would be rerouting, but if all were cut many countries would be without for a long time.
There are a few chokepoints in Europe and Asia where only a couple of fiber optic undersea cables would need to be cut to isolate a region, but for the U.S. you'd literally have to cut hundreds of cables scattered all across the Eastern and Western seaboards.
Before they all could be cut, we would notice the links going down and send in the Navy to investigate.
I suppose in your grand scheme some rogue country could spend months setting undersea charges and set them all to go off simultaneously, but there is still the undersea hydrophone network that would detect such activity and rouse even the Navy's suspicion.