Final answer:
The statement is true. When the Department of Defense is called upon to assist civil authorities, its role is supportive, reinforcing the lead of civilian agencies and adhering to the principles of civil-military relations which prioritize civilian authority over the military.
Step-by-step explanation:
When the Department of Defense (DoD) is tasked to provide military support to civil authorities, its forces indeed perform in a supporting role. This is because in the domestic context, the military provides assistance to civil authorities but does not supersede them. Civilian authorities remain in charge of the situation, with the military forces there to support and reinforce the strategies and decisions made by those civilian agencies.
The notion that the military takes a secondary, supporting role is consistent with the principles of civil-military relations, which emphasize the primary authority of civilian government over the military to ensure democratic control and prevent the misuse of military power. Therefore, the statement is True.