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...possibly by one of the classic Golden Age authors.
The basic setting is significantly in the future, set on a post-apocalyptic (class 2) Earth; the apocalypse was caused pre-story by attempted weather control IIRC.
The main character is a member of a pseudo-monastic order who use scrying powers to monitor local space for incoming hostile traffic, and as of the start of the story part of a diverse travelling band heading somewhere for some reason (which may or may not be relevant to the main plot - I forget).
Early in the story the main character detects an incoming invasion force, sounds the planetary alarm, and things develop from there.
Any thoughts/suggestions on what this might have been would be very welcome!