Standing down is a military term, which means to stop or withdraw from an aggressive stance or position. The opposite of standing down is standing up or standing tall, which would mean to advance or take an aggressive stance. Another antonym for standing down could be standing firm, meaning to remain in a stable and resolute position, usually to defend or protect something. Alternatively, standing off could be used as an antonym, referring to maintaining distance or keeping away from a potentially dangerous situation, instead of actively engaging or withdrawing from it. These antonyms, while they suggest a different approach, provide a range of options for responding to aggressive situations, whether it be through defense, aggression, or avoidance.