"Ο στρατηγός είναι ο αρχηγός στο στρατό."

[Spoonashy]

Using "in" instead of "of" is not accepted. Although I think the "of the army" construct is more natural in English, "in the army" doesn't sound odd to me and it follows the Greek grammar a bit more closely. :>

[jeanprendiville]

" The head of the army" ?

[troll1995]

That would be Η κεφαλή/Ο επικεφαλής του στρατού which may mean the same thing, but it's not the same word. ;)

[Daniela438503]

Chief is not accepted?

[Katerina182939]

Is that acceptable in english? If so, then yes, it could be used, but i had in mind that chief was mainly used with tribe, team, etc

[Jon345104]

The general is the leader in the army was marked wrong

[G.Georgopoulos]

Added.