With the united states' failure to ratify the versailles treaty, many french leaders placed their hopes for future security on

The correct answer is strict implementation of the treaty. 

Since there was no official support from the United States and its congress, there was a need to trust each other and believe that everyone would honor the treaty which didn't have the power to back it up. Unfortunately, that belief failed since Hitler rose to power and world war 2 broke out and the rest is history.