During World War II, the opportunity cost of dedicating the majority of productive resources to manufacturing weapons and other military goods, was that these resources could not be used for producing other things, not even food or other basics.
At that moment in history, even tough some economies were growing in term of absolut production figures, there was a shortage of food and people suffered from hunger.
During World War II, an opportunity cost of the decision to increase production of military goods is that these products couldn't be made into other products.
