Sorry, but I have to disagree. WW1 was rather one of the many causes that made social-security not working after WW2. The other big one being, well, WW2.
However, changing the financial scheme of social security away from a capital-backed one towards a "generational accounting" one IS what makes it not working today (unless the working population keeps it's proportional size in relation to the retirees).
Check the history of the Rentenversicherung: It was basically ruined by inflation caused by the First World War. [1]
[1] http://de.wikipedia.org/wiki/Gesetzliche_Rentenversicherung_...