A civilization that is vulnerable to attack from warring civilizations might be

The correct answer to this open question is the following.

A civilization that is vulnerable to attack from warring civilizations might be on a vast, open plain with rich resources.

That was the case of many civilizations since ancient times that were invaded by others to take their properties, lands, and exploit their raw materials. Civilizations in the valleys of what today is western Europe suffered from these attacks. The opposite happened with nations such as ancient China that remained isolated many years due to its geographical and natural barriers such as the Himalayas mountains, the Everest, the Yellow River of the Gobi Desert.

A civilization that is vulnerable to attack from warring civilizations might be

on a vast, open plain with rich resources.