How had Fort Repose become "an island"?

Fort Repose became an island because it could no longer expect any kind of assistance from outside its boundaries. The people of Fort Repose could only depend upon themselves. Like many other cities, they were cut off..... secluded.

With the Red Alert, all these services halted entirely and at once. Like thousands of other towns and villages not directly seared by war, Fort Repose became an island. From that moment, its inhabitants would have to subsist on whatever was already within its boundaries, plus what they might scrounge from the countryside.