What is the term for the reason behind an adversary’s action?

The term that best describes the reason behind an adversary’s action is "objective." An objective refers to the specific goal or aim that an adversary hopes to achieve through their actions. This could involve various motivations such as gaining access to sensitive data, causing disruption, or achieving financial gain. The understanding of an adversary's objectives is crucial for defenders as it assists in predicting potential threats and aligning their security measures accordingly.

In contrast, strategy encompasses the broader plan or approach used to achieve multiple objectives, while tactics refer to the specific methods employed in executing parts of that strategy. "Mission" typically refers to a broader sense of purpose or task assigned to an organization or group, which can include many objectives but lacks the specificity of a single objective. Therefore, the concept of “objective” is fundamental to analyzing and anticipating adversarial behavior, making it the correct choice in this context.