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.

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.