This is the 10013th most frequent Russian word.
армиями
The Russian word "армиями" means "by armies" or "with armies" (instrumental plural of "армия" - army).
The word 'армиями' is used to denote the military forces that are being led, referring to plural armies in an instrumental case in Russian.
В военной стратегии важно руководить армиями с умом.
In military strategy, it's important to lead armies wisely.
Here 'армиями' indicates the means by which the fighting was conducted, utilizing armies in the instrumental case.
Старинные короли воевали армиями своих государств.
Ancient kings fought with the armies of their states.
'Армиями' in this sentence shows possession and use, signifying the armies associated with the respective peoples, again in the instrumental case.