Fitness for command is a very general term, yet it implies a set of qualities which intelligent people easily understand and attach to the phrase.

Self-command is proverbially one of the chief.

Courage and presence of mind are indispensable.
Ability to decide and firmness to stick to a decision are necessary.
Intelligence enough to understand the duties demanded of him and to instruct his subordinates in theirs is another requisite.

But beside all these, there is a constitution of body and mind for which we can find no better name than military aptitude.

For lack of it many estimable, intelligent, and brave men failed as officers.