Dative is required by быть in this structure. (i.e. быть + noun(pronoun) )

Ex. Быть войне (dative) - The war is pending, The war is to happen.
Быть засухе - The drought is to happen (is expected).

But this structure sounds a bit old-fashioned.
The modern way of saying is Будет война - The war will happen. Будет засуха - There will be a drought.

In terms of cases any pronoun is similar to a noun. Hence чему requires Dative as any noun would in this structure.