Why did they use past perfect rather than present perfect here?
"Our enemies will soon discover that we are not as meek as they *had believed*."

What would be wrong if I switch that into "have believed"?

Question

Why did they use past perfect rather than present perfect here?
"Our enemies will soon discover that we are not as meek as they *had believed*."

What would be wrong if I switch that into "have believed"?

Answer

because you have two past actions here. They thought we were meek, and they believed it.  Normally two past actions in the same sentence require the past perfect.

Answer

When you are reporting something said or believed from some unknown period in the past, you use the past perfect, and not the present perfect.

The enemies thought/held a belief we were weak. They held this belief/thought over an unknown period of time in the past. I am reporting this, or talking about this past belief that was held over an unspecified past period of time. ----> requires past perfect.

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