The wall around Eden.

In the wake of the Death Year's atomic holocaust, an alien invasion imposes a kind of peace upon the survivors of a shattered Earth until a small community of Quakers decides to confront the saviors with their own version of resistance. The author of A Door into Ocean ( LJ 12/1/85) juxtaposes the horrors of nuclear aftermath and the persistence of human hope with rare skill and grace. Recommended for sf collections.