The primary source of a Marine's decision-making when left unsupervised is:

The primary source of a Marine's decision-making when left unsupervised stems from personal values. This choice highlights the importance of individual ethics, beliefs, and moral compass that guide a Marine's actions, especially in situations where immediate supervision or guidance is absent.

Personal values are deeply ingrained principles that shape behavior and decision-making. These values often include concepts such as honor, integrity, and commitment, which are foundational to the Marine Corps ethos. When faced with challenging circumstances, a Marine's personal values will heavily influence how they interpret situations and make choices, ensuring that their actions align with the core principles of the Marine Corps.

Military doctrine, while essential, provides guidance and frameworks for understanding tactics and missions, but it may not be sufficient when quick, ethical decisions need to be made without direct supervision. Peer pressure can influence decisions, but it does not serve as a reliable source for judgment in critical or complex scenarios. Leadership principles also provide valuable guidance for managing and leading others; however, in the absence of direct leadership, the individual’s own values become the most significant influence on their choices.