In this report we present a compositional network proof theory to specify and verify safety properties of fault tolerant systems. Important in such systems is the fault hypothesis which specifies the class of faults that must be tolerated. In the formalism presented in this report, the fault hypothesis of a system is represented by a predicate which expresses how faults might transform the behaviour of the system. To reason about fault tolerant systems, we formulate a compositional proof method based on communication histories. Soundness and relative network completeness of the method is proven. Our approach is illustrated by applying it to the alternating bit protocol.