This paper analyzes and structurally estimates a synchronization game. Agents take part in an activity and benefit from the participation of others. Coordinated actions are fruit of correlated effects as well as endogenous interactions. Standard tools applied in optimal stopping problems for continuous parameter stochastic processes are used but the processes under study are endogenized by making their distribution dependent on the participation of the group. This setup allows for identifiability and separation of correlated and endogenous influences. The model is applied to data on military records for Union Army soldiers during the American