While it could possibly happen, it would be very difficult, even if the US government (and general power in the US) was much more powerful and organized than it currently is. The US is big, and the borders between the US and Canada and Mexico are also big; there's a lot of room to set up some discreet wires or even microwave transmitters and receivers some distance from the borders. Latency would be crap at first but would get better over time. It would be harder to get a packet from the US to a European country if all trans-ocean cables were cut, but not impossible, and at that point we're probably well into WW3. The US population is also well educated as a whole (still with many offline centers of gaining technical knowledge--i.e. books) and many citizens would have strong motivations to get the internet back, there are many large concentrations of highly technical nerds who wouldn't be afraid to start wiring up their own internet (I'd love to see a wireless mesh network take over), there are lots of computers and routers and cat5 in circulation and other useful equipment as well. Many people also have printers, flash drives, and cars, it would not be hard to communicate across cities and states with a sneakernet. For a total internet blackout cutting US citizens off from other citizens (intra country) and from the world, the amount of individual and corporate cooperation with the government which the people would hate (for taking away their access to cat videos among other things) and powerful businesses would hate (for hurting their profits) is astounding.