It means that the only advantageous thing for soldiers is that they have a regulated permission to use force, if necessary, on this mission.
Explanation:
ROE cards are cards given to soldiers on missions that provide guidance on the use of force or specific military techniques that can be used in the territory in which the mission was established. In the case of the above text, when the captain says that what the soldiers have in their favor are ROE cards, he is stating that the only advantage for these soldiers is that they will be allowed to use force, if necessary.
Roman roads is an adjective phrase. Roman is an adjective that expresses nationality and modifies roads Military success is an adjective phrase. Military is modifying success.
