we can realize that even nationalize the banks won't help in this situation. Depositors wan't physical cash not because of fear of one single bank going under but because an entire country might switch to a currency that has a hugely depreciated value (making their deposits if denominated in this currency worth much less). Since the only way to protect against this is with physical currency the only way to control a run like this is to control how much physicaly curreny someone is allowed to withdraw. It should also be noted that there isnt enough physical currency to support the entire withdrawl of all Euro deposits. It just isn't physically there.