Which one is correct?
A series of policies across the DoD and Federal Government has/have been developed.
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I believe the difference depends on the context. If they policies were developed in unison as a single intentionally cohesive series, then perhaps you would want to use "has been developed". However, if the policies were developed individually and only form a series by the fact that they are relevant to one another, then perhaps you would want to use "have been developed". In this case the grammar choice can be used to further specify the meaning you intend.