There is a scientific term for entities which can only exist under pressure. Once the pressure is removed, the entity falls apart.
This term is also applied to groups where due to adversity, the group focuses on survival, on maintaining their culture.
When there is no adversity, assimilation into the prevalent culture often occurs. The group which was once oppressed becomes a component of the dominant culture, and often ceases to function as a unified entity.
During the Cold War, using the fear that the world was on the brink of extinction forced cohesion, dedication, loyalty.
Whether in the East or West, the possibility of annihilation created unity.
By the mid-60s, people began to question their governments, and the propaganda they were fed.
Societal cohesion began to break down, divisions appeared.