How do you validate a wargame model when real combat data is unavailable?

I am working on a simulation model for military engineering decision-making, and the obvious validation route, comparison against real operational data, is not available for most scenarios.

I have been relying on expert elicitation and historical case studies, but reviewers often push back on how much confidence that justifies.

How do others in the field handle validation when the ground truth simply does not exist?