The idea of collective security is that countries join together globally and agree that the security of each state is of concern to them all. Thus, each would agree to defend any state in the system that was threatened or invaded by another. Ideally, there would be no alliance systems as the global community would work together to police the system and each state would feel secure in the knowledge that it did not stand alone but could call upon the global community if attacked.
The League of Nations was designed to promote collective security and also to embody the ideal of law and institutions as the means to resolve conflicts, not the use of force. What did the League achieve, and why did it fail? (Hint: It has something to do with the US, Japan, and Italy, as well as clashing principles governing security).