This is the 10619th most frequent Russian word.
военнопленными
"Военнопленными" means "prisoners of war" in English.
Here, 'военнопленными' is used in the instrumental plural to indicate the subject being referred to under the given treatment.
Обращение с военнопленными должно соответствовать международным стандартам.
The treatment of prisoners of war must comply with international standards.
Here, 'военнопленными' is used as a noun in the instrumental plural form, referring to soldiers who are in the state of being prisoners of war.
Во время войны многие солдаты оказались военнопленными.
During the war, many soldiers became prisoners of war.
In this sentence, 'военнопленными' functions as an instrumental plural attribute describing the liberated individuals.