Nuclear war wipes out over half of the cities in the 1980's.
With every telephone line destroyed and the telephone infrastructure destroyed by EMP, the world is isolated. In the struggle to survive and remain in contact with each other, telegraphs are put in place as a stop gap measure.
Amongst the few surviving military bases of North America, the internet survives. Using it the military begins to coordinate and attempts to rebuild civilization. To help with this task, the few remaining computer programmers create an instant messaging program allowing the generals to remain in constant contact.