This is the 3110th most frequent Russian word.
командующего
"Commanding" or "commander" (genitive or accusative case).
The word 'командующего' here is used in the masculine singular genitive case because it reflects possession ('of the army').
Командующего армией пригласили на совещание.
The commander of the army was invited to the meeting.
Here, 'командующего' is in the masculine singular genitive case, indicating the source from which actions or directives originate ('from the commander').
Все подчиненные ожидали указаний от командующего.
All subordinates were waiting for orders from the commander.
In this sentence, 'командующего' is in the masculine singular accusative case, as it is the direct object of the verb 'поздравили' ('congratulated').